' Besos, guapa! ' means "Kisses, [my] beautiful [one]". You could answer: ' Gracias! Para tí también, muchos besos, guapísimo. ' > "To you too, many kisses, handsome one".
Guapa is a feminine adjective meaning pretty or good looking. La chica es guapa. (The girl is pretty.) Guapo, it's masculine counterpart, means essentially the same thing: Handsome or good looking.