im also autistic, but i would do anything for my grandma

Sad, no matter how you look at it. Either OP has such an extreme case of autism they cannot tolerate grandmothers party or they just don’t want to stress themselves. Either way is understandable, but I can see how your absence might be misinterpreted. I assume Grandma is already aware of OP’s autism and what that means. So she should be able to understand why they might skip the party. Talk to her and wish her happy birthday and tell her you wish you could be at the party.

Did you explain to her why you were missing her birthday party?

Maybe not hate, but she's right about her not being a priority for you. Hopefully, she's not too close to death and you can get your shit together to weasel your way back into her will.

I would think your mom or dad would have explained the situation to her...

I have serious distress at parties too. Instead of simply skipping, I set up a day before or after to celebrate with them. It makes my family members feel loved and appreciated, while allowing me to avoid situations that make me feel overwhelmed and drained. Or you can skip entirely, but don't expect them to give you their time/money when you won't do the same for them

You can tell her you think she hates you because she puts her selfishness first.

Have you tried sound-blocking ear buds? They make ones looking like accessories - these should suit well at parties.

Get some ear plugs or ear muffs for occasions like this. They can really help, also go visit gma and bring her something special.

maybe just take her out to a quiet lunch instead and if she doesn't already know explain to her why it was difficult for you. a good family member should understand