The 2015 models with dedicated GPUs had an AMD Radeon R9 M370x, so that's what you'd want to look for. That said, honestly, it's a pretty bad gaming GPU these days. What games in particular would you be looking to play?
Frankly I also just don't think it's worth it to spend any significant amount of money on a 2015 Mac at this point. We're talking a four year old laptop with a six year old processor, a sub-Xbox One caliber GPU at best, etc. Mac die-hards who hate the new design will swear by them until Apple makes a new MacBook that satisfies them, but I couldn't recommend it to average users. If you really don't want a new model and you can't find a really good deal on an older one, either wait for a redesign or look elsewhere.