Probably depends on the country. I know that in Germany it is legal to include "public" people, parody or not (that is all information to the best of my knowledge, don't take it for granted). So you could absolutely use Lady Gaga and Slash, just not use any photos somebody else has taken of them but instead draw them yourself I guess. Even though, realistically I'd say just go ahead. Nobody will sue over you for including a star like that in an AGS game.