The Lions, who are in Jersey on a training camp, leave for South Africa after Saturday's match at Murrayfield and Gatland has promised that every player in his 37-man squad will start a game before the First Test on July 24.
The team for the Lions' first-ever fixture against Japan — which will be played in front of a crowd of 16,500 — is captained by Alun Wyn Jones. He is one of five Welsh players in a side that also features six Irishmen and the four Scots. Props Rory Sutherland and Zander Fagerson, flanker Hamish Watson and wing Duhan van der Merwe start, with Ali Price on the bench.
There is no place for centre Chris Harris, with Robbie Henshaw and Bundee Aki given first crack at that position. Other striking Irish selections include Jack Conan at No8 ahead of Taulupe Faletau and Tag Beirne at blindside.
There are no English players in the starting line-up, but Saracens' Jamie George and Owen Farrell have been rushed into action from the bench after they secured the club's return to the Premiership on Sunday.
Lions XV vs Japan
15 L Williams; 14 J Adams, 13 R Henshaw, 12 B Aki, 11 D van der Merwe; 10 D Biggar, 9 C Murray; 1 R Sutherland, 2 K Owens, 3 Z Fagerson; 4 I Henderson, 5 A W Jones (capt), 6 T Beirne 7 H Watson; 8 J Conan. Reps: J George, W Jones, T Furlong, C Lawes, T Faletau, A Price, O Farrell, A Watson
Ken Owens and Dan Biggar have been the only hooker and fly-half in the Jersey squad, meaning George and Farrell were called into action immediately.
Anthony Watson and Courtney Lawes were the only Englishmen in the Jersey party as the domestic season reaches its conclusion and both are named on the bench. Ten of the 23-man squad will be making their first Lions appearances.
Meanwhile, Manu Tuilagi has withdrawn from England's squad for the summer Tests through injury.