About Sonic Gems Collection

Streets of Rage 1, 2 and 3 and Bonanza Bros. are included only in the japanese version of the game.
Vectorman 1 and 2 are included only in the european version of the game (in order to obtain a lower ESRB / BBFC rating).

The European version uses the american soundtrack for Sonic CD (whereas the original European Mega-CD game used the japanese soundtrack).