Mon gars means something like guy, chap
vous êtes mon meilleur ami > you are my best friend 'un gars' - plural 'des gars' means "guy / guys" in French.
Mon patron is "my boss" in French. In French, the word is never used to mean 'customer'.
It means "my little" as in "my little one".
a pal = un copain (pronounce cop-ann) Hi matey, Hi chum = Salut, mon gars
mon (+ masculine noun) or ma (+ feminine noun)
mon Dieu means 'my God' in French.
'mon fils' means 'my son' in French.
mon cadeau means my present / my gift in French
Mon oreille (feminine noun) means "my ear" in French.
mon père - my father
You can say "mon frère" to mean "my brother" in French.
The French phrase "mon frère" means "my brother" in English.