I'm a trans woman and my girlfriend started dating a TERF

I'm a trans woman who has a penis repulsed girlfriend. Not having sex together was fine but we both got dissatisfied eventually and tried to open up the relationship (I'd been poly in the past so it wasn't a problem). She is new to poly and just recently began connecting with an older lesbian. I found out this older lesbian woman is a huge TERF after seeing screenshots in a trans discussion thread. Should I tell my girlfriend that this person she REALLY likes doesn't believe in my existence?

Honestly, that's not a call I can make for you. If you're uncomfortable knowing that someone in your social orbit is a TERF and feel you need to check in with your girlfriend about this, that's totally okay. Be careful to be gentle and non-accusatory - she may feel like you went out of your way to find reasons to criticize or sabotage her new relationship, which isn't true, but can often be the "message received" when an existing partner has something negative to share about a new metamour. Or, she may have an explanation, more context, or other input that can add nuance to the situation.

Also, go into it with a proposed solution or an articulation of your needs - do you want your partner to stop dating this person? Do you want your partner to commit to keeping this person away from you? Do you not mind meeting or being around this person, but want your partner to be aware that you need her to speak up with confidence if anything TERFy or transphobic is said? Be clear with her why you're telling her this and what you need moving forward.

Say something like "hey, I know this really sucks and puts us both in an awful situation, but in one of my trans discussion groups, someone posted screenshots of Garlinda saying transphobic things. I'm worried about having this person in my life because she might say or do something that could really hurt me. Have you two talked about me? Has she said anything about your existing relationship that concerns you? Do you think that if she says anything transphobic, you'll be willing to speak up about your intolerance for that mindset?" Then give space for her responses and strategize together about how to balance her freedom with your safety.

That said, you're not obligated to stay in a situation that puts this woman in your orbit. If your girlfriend will not commit to doing what you need her to do to keep you safe, or if she gets angry and defensive and insists that you have no right to be concerned about what you saw, or if something happens in the future with this woman that makes you uncomfortable or upset, you don't need to stick around and tolerate that.