user105: I have observed that the King's Gambit is usually not played at international level.  Is there any specific reason for this?

user2076: Typically, White wants to play the King's gambit to play some crazy lines, full of sacrifices and double-edged play like the Kieseritzky Gambit:


    [FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

    1. e4 e5 2. f4 exf4 3. Nf3 g5 4. h4 g4 5. Ne5 Nf6



With a messy position on move 5 already !
And there are so many lines like that if Black is ready to accept the challenge. But that is a big IF.
First, there is this defense advocated by Fischer everyone is mentioning that seems to be OK for black:


    [FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

    1. e4 e5 2. f4 exf4 3. Nf3 d6



This move d6 simply removes the e5-square for the Knight, and this alone makes White's attack harder to carry on.
But I think that, really, what can discourage White to play the King's gambit, especially at higher level, is that Black can simply opt for an early d7-d5 setup, that usually gives a full (and quite boring) equality:


    [FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

    1. e4 e5 2. f4 d5 3. exd5 exf4



And honestly, after these 3 move, it is hard to see how White can attack...
Check my introduction on the King's gambit if you want to get an idea on how to play this nevertheless interesting opening.

user15223: Top players don't play sharp gambits such as this one much because they involve lots of forced lines. These lines are easy to calculate for an engine. The result is that you no longer play against your opponent - you're pitting your engine / memorization skills vs. theirs, and not your chess ability vs. theirs.
Source: Romantic chess in modern times


user734: One item that is not a reason that the King's Gambit isn't played more at the international level is this specious article from Chessbase News: "Busting the King's Gambit, this time for sure." In the article, Vasik Rajlich, the author of Rybka, claims that all moves but 3.Be2 lead to a forced win for Black, while that move draws. However, though this was posted on April 2, 2012, rather than on April 1, the article was in fact one of Chessbase's annual April Fools' posts.

user12: The King's Gambit is considered inferior because white is sacrificing king safety along with a pawn on move 2.  Furthermore, although white would get an attack going along the f file in Morphy's day, now the correct defensive ideas are known for black and white has a harder time of it.
That said, the King's Gambit is a great weapon at the club level.  In fact, Quality Chess will be releasing a book on the King's Gambit very soon.  

user657: David Bronstein, in his book "The Modern Chess Self-Tutor," gives an interesting explanation for not playing it. Don't forget, he's used it many times in GM practice, including memorable games against Tal and Petrosian (the latter was an exhibition game in which they explore more fully an idea from an Alekhine-Keres game).
His reasoning for 2. Nf3 being preferable to 2. f4 is that like 2. d4, 2. f4 fully declares White's intentions too early in the game. White is better off playing something like 2. Nf3, when Black still has no idea what sort of game White is aiming for. He doesn't think it's necessarily worse, just that playing it makes Black's choices easier. 2. Nf3 keeps White's options open.

user101: One of the more common lines for the King's Gambit is accepting, followed by 3. Nf3.  That is, 1. e4 e5  2. f4 exf4 3. Nf3.  After this, black can follow with 3... d6, 
the so called Fischer defense.  Fischer was extremely confident that this was a bad position for white.  He wrote an entire article about this line, and famously said, "In my opinion the King's Gambit is busted. It loses by force."  On top of this he added, "Of course White can always play differently, in which case he merely loses differently."
I am not sure about other lines, but likely top grandmasters have analyzed them, and come to similar conclusions that white's position is not a good one.
Remark: Interestingly Fischer himself played the gambit a few times with great success, but he opened 3. Bc4. 

105: <p>I have observed that the King's Gambit is <em>usually</em> not played at international level.  Is there any specific reason for this?</p>
 2076: <p>Typically, White wants to play the King's gambit to play some crazy lines, full of sacrifices and double-edged play like the Kieseritzky Gambit:</p>

<pre>
<code>
    [FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

    1. e4 e5 2. f4 exf4 3. Nf3 g5 4. h4 g4 5. Ne5 Nf6

</code>
</pre>

<p>With a messy position on move 5 already !
And there are so many lines like that if Black is ready to accept the challenge. But that is a big IF.
First, there is this defense advocated by Fischer everyone is mentioning that seems to be OK for black:</p>

<pre>
<code>
    [FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

    1. e4 e5 2. f4 exf4 3. Nf3 d6

</code>
</pre>

<p>This move d6 simply removes the e5-square for the Knight, and this alone makes White's attack harder to carry on.</p>

<p>But I think that, really, what can discourage White to play the King's gambit, especially at higher level, is that Black can simply opt for an early d7-d5 setup, that usually gives a full (and quite boring) equality:</p>

<pre>
<code>
    [FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

    1. e4 e5 2. f4 d5 3. exd5 exf4

</code>
</pre>

<p>And honestly, after these 3 move, it is hard to see how White can attack...
Check my <a href="https://pawnbreak.com/getting-started-with-the-kings-gambit/" rel="nofollow noreferrer">introduction on the King's gambit</a> if you want to get an idea on how to play this nevertheless interesting opening.</p>
 15223: <p>Top players don't play sharp gambits such as this one much because they involve lots of forced lines. These lines are easy to calculate for an engine. The result is that you no longer play against your opponent - you're pitting your engine / memorization skills vs. theirs, and not your chess ability vs. theirs.</p>

<p>Source: <a href="https://en.chessbase.com/post/romantic-chess-in-modern-times" rel="nofollow noreferrer">Romantic chess in modern times
</a></p>
 734: <p>One item that is <strong>not</strong> a reason that the King's Gambit isn't played more at the international level is this specious article from Chessbase News: <a href="http://chessbase.com/newsdetail.asp?newsid=8047" rel="noreferrer">"Busting the King's Gambit, this time for sure."</a> In the article, Vasik Rajlich, the author of Rybka, claims that all moves but <code>3.Be2</code> lead to a forced win for Black, while that move draws. <em>However, though this was posted on April 2, 2012, rather than on April 1, the article was in fact one of Chessbase's annual April Fools' posts.</em></p>
 12: <p>The King's Gambit is considered inferior because white is sacrificing king safety along with a pawn on move 2.  Furthermore, although white would get an attack going along the f file in Morphy's day, now the correct defensive ideas are known for black and white has a harder time of it.</p>

<p>That said, the King's Gambit is a great weapon at the club level.  In fact, Quality Chess will be <a href="http://www.qualitychess.co.uk/products/1/124/the_king" rel="noreferrer">releasing a book</a> on the King's Gambit very soon.  </p>
 657: <p>David Bronstein, in his book "The Modern Chess Self-Tutor," gives an interesting explanation for not playing it. Don't forget, he's used it many times in GM practice, including memorable games against Tal and Petrosian (the latter was an exhibition game in which they explore more fully an idea from an Alekhine-Keres game).</p>

<p>His reasoning for <code>2. Nf3</code> being preferable to <code>2. f4</code> is that like <code>2. d4</code>, <code>2. f4</code> fully declares White's intentions too early in the game. White is better off playing something like <code>2. Nf3</code>, when Black still has no idea what sort of game White is aiming for. He doesn't think it's necessarily worse, just that playing it makes Black's choices easier. <code>2. Nf3</code> keeps White's options open.</p>
 101: <p>One of the more common lines for the King's Gambit is accepting, followed by <code>3. Nf3</code>.  That is, <code>1. e4 e5  2. f4 exf4 3. Nf3</code>.  After this, black can follow with <code>3... d6</code>, 
the so called <a href="http://en.wikipedia.org/wiki/King%27s_Gambit,_Fischer_Defense">Fischer defense.</a>  Fischer was extremely confident that this was a bad position for white.  He wrote an entire article about this line, and famously said, "In my opinion the King's Gambit is busted. It loses by force."  On top of this he added, "Of course White can always play differently, in which case he merely loses differently."</p>

<p>I am not sure about other lines, but likely top grandmasters have analyzed them, and come to similar conclusions that white's position is not a good one.</p>

<p><strong>Remark:</strong> Interestingly Fischer himself played the gambit a few times with great success, but he opened <code>3. Bc4</code>. </p>
