How to deal with being clocked

My girlfriend was dmed by a friend of hers and someone I knew and was chill with as well yesterday asking if I’m trans and how she “needs to know”. Idk how to tell my gf to respond because it feels like anything (feigning confusion, denying, saying it isn’t the friend’s business) could be suspicious and confirm it to them. A potentially important note is this person who asked her is queer and I think has a trans partner, so I know it isn’t malicious but it still genuinely uneases me to think I was some kind of topic of discussion in this way especially since I hadn’t seen this friend in a pretty long time.

I’m stealth and even if I may not do a good job at it (I’ve been asked before a year ago at this same school if i was trans) i still do want to maintain denial in the least suspecting way possible.

How would a girlfriend normally react to someone asking if their cis boyfriend is trans?