I'm going to blow up at my gf because she ruined my Christmas gift surprise to her
Oh no, she actual ruined her own surprise? How so?
I gave her my guitar necklace with a card, she opened it and said she hates guitars because of her father.
It sounds like your girlfriend is being ungrateful. You tried your best to surprise her and she is being very rude. She should be more supportive.
That makes me more mad. She should be grateful for what I did for her.
Maybe she just did not know how to react to it. You should explain to her the heartfelt sentiment behind it and maybe she will be more appreciative of the gift you gave.