It's a really tough call. Current GF knows, is a competitor and uses various stuff, T3/Clen/DNP and others, so we're even and there's no stigma. This is the only way to go, IMO.
With a "non-lifestyle" GF, you can hide it, but you'd have to commit to total secrecy and a protocol of use that you NEVER break. Problem is, you'll get lazy and slip up, leave something out, hit a vein and leave a bruise a couple times in a couple weeks, get a pack delivered when she's home. It's a constant worry, like any repeated lie in a relationship. The truth will out, it just will.
But...if you choose to tell her/she finds out...it will be an issue EVERY time she's pissed or after you break up.
I never had that problem until an ex became a stalker. She showed up and freaked out as she did periodically. When she calmed down, she asked me all nonchalant, if I was still using gear. I told her, "Hell no, it's been months and I've cleaned house. I'm done with that shit". I KNEW she was gonna set me up. I could just feel her bad intentions...So I did clean house. Because I knew if I called the police on her sometime, they'd be in my house the same evening on what she told them. And in Europe, we have no rights. Good times.
Honesty is nice, but it does leave you open to problems.