I think it really depends on how important a concern this is for you. I can only relate to my own experiences. (I should mention I've only seen a few therapists for brief periods over the years.)

For me my Gender Dysphoria (plus some other related stuff) is such an integral part of who I have been, as well as what my experiences have been, that to not bring this stuff up made seeing a therapist pointless. And that is, perhaps, why I tried a number of different therapists & found seeing them pointless, because I was too embarrassed to talk about these things for the most part. However, for you, perhaps it's just not that big a deal?

I like Lostislost's suggestion for introducing the subject though. Just in passing mention you saw your gf, perhaps along with a bit about what the two of you did, & then carry on from there with whatever else there is to be discussed. Your therapist may well pick up on that & ask if you want to talk more about it which you can then do, or not do, as you prefer. You can also mention your gf again during subsequent appointments when there's an appropriate moment. That way it doesn't come across as some big deal that needs to be grappled with. It's simply another aspect of your life you're mentioning here-&-there as your appointments continue. Perhaps it will lead to something important... perhaps not. But either way it's a significant aspect of your life you've allowed into your conversations with your T. At least that's the way it seems to me.