According to a Japanese retailer blog, Sonic Gems Collection contains a number of hidden games. The only game listed in the blog entry is the MD version of Bare Knuckle (i.e. Streets of Rage), but it does say there are quite a few hidden titles. There's a possible link between the Knuckles in Sonic and Bare Knuckle too, but we don't have much to go on either way, regarding the other titles.
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