The sequences αυ, ευ, ηυ are /av ev iv/, or /af ef if/ before a voiceless sound; the sequence ου is /u/; and otherwise υ is /i/.
(ηυ is rare, especially in modern Greek. I think it only occurs as the result of augment, which modern Greek usually doesn't do any more.)
You may be confusing two things: words and things that the words stand for.
Girls are female.
The word κορίτσι is neuter.
The word refers to a girl, and girls are female, but the word is not feminine.
Word gender is more or less arbitrary. Things that refer to females need not be grammatically feminine, and vice versa.