As the others said, we'd need to know more about your intents.
But I will tell you how I would go with this. First, here's a bit about me. I absolutely do not believe in monogamy nor try to practice it. I think it's kinda foolish to try monogamy for the simple reason that vast majority of people are trying and blatantly failing. If you've ever played poker before, you probably don't wanna bet on a J-4... but hey some people do!
So, if a chick asked me the "What are we?" question, then my answer would obviously depend on how long I been hanging out with her and how interested I am.
1. Under 6 months, but I am interested in a relationship (never exclusive, of course) with her or just not decided yet, then I would tell her something like this;
"I really like where this is going and am looking forward getting to know you better and am glad I met you!"
2. Over 6 months, definitely interested;
"I absolutely love where this is going and am truly glad we met and can't wait to see what the future has for us!"
3. Time irrelevant, only interested in FB;
"I absolutely love what we have right now and I wouldn't want to change anything and can't wait to see what adventures await us!"
I would highly recommend against saying things like "Do you want to be my gf?" or "I guess you are my gf?" because you are giving up frame when doing so, and she might just respect you less for it. Nothing much other that short term happiness can come out of this. You want to be covert and indirect but be positive and nice about what you say to her.
If she would ask if she can be my gf, then this is trickier because she's pretty much asking if you want to give her exclusivity? We understand women are very rarely if ever direct or overt in how they communicate, so you have to read between the lines.. I do not recommend asking for clarifications as too many times the logic will make them upset and/or confused.
1. Under 6 months, but interested or unsure;
"Holy cow lookat chu! Quite the speedy gonzales aren't we?
**be funny and sarcastic!** I don't even know your favorite color or safety word!! I really like where this is going and am looking forward getting to know you better and am glad I met you tho!
2. Over 6 months, definitely interested;
You can call me whatever you want! Would it change anything tho?
I absolutely love where this is going and am truly glad we met and can't wait to see what the future has for us! I think I'll start calling you *insert your own sweet name*
3. Time irrelevant, only interested in FB;
How about you call me your pet? or... oh wait.. God? Ya I like that one!!
Some guys (mostly blue pilled) will say "Just be yourself" or "Just say what you really think" or "Just say the truth" but I call bull**** on that... that's the kinda advice a women would give you!! And we all know they'll just say whatever serve the feminine imperative better.
Women are constantly subconsciously testing how stern and rigid the dude they're dating is. You want to be very careful in how you respond to their questions and I find that usually simply saying "Yes or No" or what's really on your mind (rationale, logical reasoning) will do more harm than good. You want to be very subtle in what you say, and women will understand that because this is their preferred method of communications. They do not really want to know... they mostly care about the image.
If you say "No, you are not my gf" then it'll look bad for them if they stick around but if you say something like my answers to #3, then she'll understand what you are saying but she won't think it affects her image... yet anyways. They want to rush things, you want to buy time but it is indeed tricky to not send negative vibes but also not tarnish your own image or attractiveness.
Too many guys out there fail to understand that if they answer all the right answers to a women, they're gradually depreciating in value in the woman's eye. By agreeing to her being your exclusive gf after 2 months, this is exactly what you are doing. It doesn't make any sense because it is rushed and she knows it but she wants to see if she can secure a provider for her own self. And most women do not really value providers very much... they might for a bit... until Chad shows up
Be Chad...!