Nouns ending in -in are usually masculine.
(Words ending in -ain, -ein, -oin, -aim, or -uin follow different rules.)
Is gamin masculine or feminine?
legamin
The feminine form is gamine
The rule behind it
97.4% of -in nouns are masculine
In context
Je ne sais pas qui est ce gamin.
I don't know who that boy is.
Tu agis comme un gamin de 3 ans.
You're acting like a three-year-old.
Le gamin est grand pour son âge.
The boy is tall for his age.