I'm going to call out my gf's sleep farting
How do you think you'll go about doing that?
I might be able to do it in a joking way
As long as you're not too mean about it, it's fine to bring up the sleep farting.  You don't want to be insulting though, as you should be nice to your partner.  Otherwise you might cause her to feel bad about doing it.
Okay, I will think about it
Your solution sounds like it will work and she'll appreciate the humor. It is good to see your willingness to please others while also not accepting bad behavior from others.