Also fully restored to the squad, which has been cut from 32 in the Autumn, is Ospreys hooker Huw Bennett. Cardiff Blues centre Jamie Roberts is named having completed his recovery from a fractured skull sustained in the win over Australia two months ago.
"There are a few important players back from injury and we think they are going to really strengthen the squad," said Gatland.
"We had a very hard decision to make at scrum-half, but were greatly impressed by the performances of Mike Phillips last year during the Six Nations and felt Gareth Cooper did well both on tour in the summer and in the Autumn Series.
"It's a decision that we debated long and hard and the result is really difficult. It probably boiled down to the fact that Mike Phillips has played more rugby than Dwayne Peel since the Autumn, but it doesn't mean that the door is always going to be shut for him or anyone.
"We've tried to keep the squad relatively similar to the last Six Nations campaign. Part of that philosophy is to keep everything nice and tight and everyone in the squad feeling there is an opportunity to play for Wales.
"We also want to make it difficult for people to come into the squad, making players really earn the right."
Wales launch their defence of the title against Scotland at Murrayfield on Sunday, 8 February and Gatland knows the pressure will be on his players and his coaching team.
"We will accept the challenge of being favourites. It's a new experience for a lot of coaches as well as players but, hopefully, it will be something heaped on us more regularly and be something we learn to handle," said Gatland.
"We have to make sure we are mentally right for Scotland. We have different pressures on us - a fear of failure and an expectation to go out there and perform.
"But it will be very much like last year - win the first game and create momentum for success. If we lose that first game then we come home to England and then go away to France, and it could be a completely different story."
Wales Six Nations Sqaud
Lee Byrne (Ospreys), Mark Jones (Scarlets), Leigh Halfpenny (Blues), Shane Williams (Ospreys), Tom Shanklin (Blues), Jamie Roberts (Blues), Andrew Bishop (Ospreys), Gavin Henson (Ospreys), James Hook (Ospreys), Stephen Jones (Scarlets), Gareth Cooper (Gloucester), Mike Phillips (Ospreys).
Gethin Jenkins (Blues), John Yapp (Blues), Adam Jones (Ospreys), Rhys Thomas (Dragons), Matthew Rees (Scarlets), Huw Bennett (Ospreys), Ian Gough (Ospreys), Alun-Wyn Jones (Ospreys), Luke Charteris (Dragons), Bradley Davies (Blues), Ryan Jones (Ospreys, Captain), Jonathan Thomas (Ospreys), Dafydd Jones (Scarlets), Andy Powell (Blues), Martyn Williams (Blues), Robin Sowden-Taylor (Blues)