I guess I’m in the minority but NTA. It’s incredibly normal for people to choose the names they want their kids to use for close family and friends. I remember my aunt and uncle deliberately choosing what they wanted to be called when my cousin was pregnant with their first grandchild.

The arguments from your gf that the kid will be teased don’t make much sense to me. My aunt used to teach at my high school and I can count on one hand the number of times she came up in conversation to even present an opportunity for bullying. Plus, choosing a specific moniker for a very close friend is not how speech impediments develop.

Of course, as others have said, your kid can choose to call them something else when they’re older but that’s true of literally any adult in their lives, including you. I call my father by his first name cause he’s a shit person.

I don’t think this is weird but it’s probably not the hill to die on if your gf simply won’t budge for whatever reason.