On those senteces the meaning is the same but "gentil" is little bit more formal. The most part of the time when you use gentil you can replace it by sympa if you want but you can't replace sympa by gentil everytime. "Sympa" can mean "cool" too, for exemple you can say "j'ai regardé une série sympa hier" and not "une série gentille". "C'est cool" can be "c'est sympa"