If you were the one that brought up her birthday and you asked her what she would like as a gift and she responded with her $900 gift, than that's one thing. However, if SHE brought up her birthday herself and brought up what she wants as a "gift" than read on, OP.
She’s gaslighting you. Be clear. It’s really a bribe of sorts that is falsely being renamed as a gift.
Gifts are given, not EVER requested let alone demanded from another! Her doing so speaks volumes about her character, or more accurately I should say lack there of.
She has not been (if I recall correctly) a stable, happy, drama free, loving, caring GF of 1 year. I think there has been a fair amount of push pull and drama between you two, if I recall correctly. I could be wrong and mixing you up with another. Factor in what has actually been the truth of the dynamics between the two of you because It is important.
Consider the very real possibility that she could/may very well ghost you at any point, once she receives your $900 "gift." Will you truly be ok with that? Only give it if you are truly ok with receiving NOTHING in return. You will have no recourse. After all it is a “gift.” She will owe you nothing. Prepare yourself for the possibility that she may disappear shortly after.
On the other hand, if she does stay after receiving your “gift,” expect that sooner rather than later, she will dig in and raise the ante, even higher. Then rinse and repeat.
This is NOT a woman that genuinely cares about you, OP! Don’t lie to yourself about that. Don’t pretend or try to justify otherwise. She is a woman cares about what you can do FOR HER, and how she can best make that happen, FOR HER.
Best gift you can give her, IMHO, is your abscence. She doesn’t deserve you. You can become seared into her memory as the Man that couldn’t be bribed/manipulated, with her pvssy or presence. She won’t lose any sleep if you move on. She will simply and quickly move on to the next man that will pay her current asking rate.
She’s offering you a transactional relationship/interactions at best. Nothing more. That may work for you and be acceptable. Some men utilize transactional set-ups with women. Certainly do NOT confuse it with her actually caring about YOU. She does NOT! Women who genuinely care about their man, don’t demand certain “gifts” (in EXCHANGE.)
Should you decline to give into her demand/s, whether she ever admits it or not she will always respect you for that choice. A woman doesn’t truly respect a man she can manipulate.
Should you give in to her demands know that she will lose respect for you, and with it any true sexual desire/passion she has had for you will decline as well. Sex will become more and more transactional (less passionate) as she begins to see it and calculates it as part of what she has to do, to get what she wants from you.
She has revealed more of her true colors to you now. Pay attention!
No matter how good she may look, or how hot she is, I think you are better served taking your self respect and moving on. That's me.