Portugal forward Ronaldo left United for Real Madrid in 2009 for a then world record £80m fee and has become the Spanish side's top goalscorer.
However, speculation persists that the 30-year-old is set to leave Madrid.
"We are looking at all players, not just Ronaldo. But these players are mostly ungettable," said Van Gaal. "With Ronaldo, let's wait and hope."
Ronaldo, a three-time Fifa world player of the year, broke Raul's record of 323 goals for Real earlier this season.
He has won one La Liga title, a Champions League crown and two Spanish Cup titles in just over six years in the Spanish capital.
Before moving to Madrid, he won three Premier League titles, one Champions League, one FA Cup and two League Cups in six seasons at United.
He moved to Old Trafford for £12.24m from Sporting Lisbon in 2003 and scored 118 goals in 292 appearances for the Red Devils.
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Manchester United manager Louis van Gaal has confirmed he wants to take Cristiano Ronaldo back to Old Trafford.