There are different possible translations for guy in French. Those are
- Mec : most familiar of the four solutions
- Type : designates a rather ordinary personne. So someone you would't know mostly. (e.g. Ce type dans la rue -> This guy in the street)
- Gars : this one would be the most appropriate for your case, it is familiar but not that much. Can be used with friends (e.g. Allez les gars ! -> Come on guys !)
- Homme : a better translation would be man, it is the most formal of the four.
- Monsieur : highly formal. (e.g. Ce monsieur assis au bar -> This gentleman sitting at the bar)
as for the feminine equivalent gal:
- Nana, gonzesse, nénette : familiar words that are falling out of usage
- Meuf : familiar also, this is the verlan (reverse) version of the word femme that may be one the most used by people under 30 but can only be used orally.
- Fille : not familiar at all, although you would not use it in front of the person you are talking of. You could however say : Cette fille là-bas ! which would be translated by : That girl over there!
- Femme : most formal alternative that would be translated by woman rather than gal
- Dame : highly formal, equivalent of the english lady
Finally for both men and women, one could use :
- Personne : this is the non-genred version of man and woman. This is formal but less than dame or monsieur
As Laure suggested you can also use the words celui ou celle although you wouldn't translate this to guy or gal but rather to the one (e.g. Celui avec qui je vais au cinéma -> The one with which I'm going to the movies)