The 33-year-old missed out on the original squad but was called up after injury ruled out Rhys Webb.
Asked if Phillips was behind Gareth Davies and Lloyd Williams, head coach Gatland replied: "I don't think so.
"We were happy with our selections but Mike coming back gives us experience and gives us a different type of game if we want to go to that."
Phillips has played 94 times for Wales and five times in Tests for the British and Irish Lions, so is poised to make his 100th international appearance during the tournament.
Gatland said the manner in which the player had handled being left out of the squad had been exemplary.
"I thought the way he conducted himself after being left out was fantastic," said the New Zealander.
"He sent a nice text to Lloyd Williams saying 'you've retired a legend' which I thought that was brilliant.
"He's come back in and we know what a competitor he is, but he's just kept his head down and having his confidence and self-belief in the squad as well - that's something he doesn't lack - it's going to be positive on the other players."
Wales kick-off their Pool A matches against Uruguay in the Millennium Stadium, Cardiff, on Sunday, 20 September.
Gatland names his team on Friday, but confirmed long-term injury dobuts prop Samson Lee and full-back Liam Williams will play some part but "probably not 80 minutes" against the South American minnows.
After Uruguay, Wales play England in Twickenham on 26 September and Fiji in Cardiff five days later.
"It's about looking after one or two players in the first game and thinking about the following week and the Fiji week as well.
"We want a good performance and to get as many points as we can, though not disrespecting Uruguay in any way.
"It will be their first game and they'll bring everything to the table too."
Mike Phillips is not being regarded as Wales' third-choice scrum-half for the World Cup, according to Warren Gatland.