I'm from the
UK.
Gemma is dated here, it's a very 80s name. It's not seen as particularly low class, but it's definitely not upper class either. I'd put it in the same bracket as names like
Laura,
Stephanie,
Hannah,
Lisa and
Rachel.
I prefer
Jemma (this how my cousin spells it) to
Gemma, and
Emma to Gemma/Jemma.