When used as a first name instead of a nickname, Gaby is, according to most naming sites/book, a masculine name. It is short for Gabriel, which means "strong man of God" in Hebrew.

Since my brother is the one who suggested my name, he is to blame for naming his little sister with a guy's name. I have considered changing it to "Gabrielle" legally, but it's expensive, there's too much paperwork involved, and one of my mother's brothers is named Gabriel.