Williams, who finally won a Lions Test cap last year following previous tours in 2001 and 2005, had shoulder surgey after returning from South Africa with the Lions and it is thought he will benefit from a summer off as Wales build towards next year's World Cup.
Sale star Peel had been hoping to regain his No9 shirt from fellow Lion Mike Phillips but he is deemed to need rest, recuperation and a solid pre-season after the well-documented injury problems that have blighted his campaign.
Hook, who starred for Wales in an unfamiliar role at outside centre during the Six Nations, will have shoulder surgery as soon as the Ospreys' season comes to an end and will therefore be unavailable to tour.
Newport Gwent Dragons lock Luke Charteris is another player to be ruled out as he attempts to recover from a neck problem.
"We have some depth in these positions with the likes of Jamie Roberts, Andrew Bishop and Jonathan Davies playing well at the moment at centre," said 2009 Lions forwards coach Gatland, whose Wales team face South Africa in Cardiff on June 5 before taking on the All Blacks in a two-Test series.
"We are okay in the second row as well, and Sam Warburton has been pushing Martyn really hard for a back row spot at the Blues as well as at international level.
"South Africa in Cardiff, and then two Tests in New Zealand, means we have a tough summer ahead of us. But this is exactly the position we need to be in and it will be perfect preparation for the World Cup.
"Games don't come much tougher than playing the world champion South Africans and Peter de Villiers has a wide range of talent to choose from.
"Depending on what happens in the Super 14, certain players will be ruled out. But we will be expecting the European-based players like Butch James to figure and Sharks players are likely to be available, so there will be World Cup winners throughout the squad.
"They are going to be strong, but it could also be a great day for Wales. If we could win that game it would be a great platform for our summer tour.
"As for New Zealand, we just need to believe in ourselves and perform like we know we can. Then we will have a chance. France won a game there last year and Ireland ran them close as well, so we know it can be done.
"But we are also keen to familiarise the players with our World Cup venues and training bases. It's not until I left New Zealand that I realised that it is such a difficult place to tour and we want the players to have that experience, training at the same venues and staying in the same hotels as we will in 2011."
Gatland will name his tour squad this Sunday.