I don't want to use a gift my grandparents got for my birthday
Why do you not want to use it?
The gift is not something that I want or can use
You are behaving in a selfish way and need to be more grateful for the gifts that you receive. If your grandparents feel that you are not being grateful for what they give you, they may not give you any gifts in the future.
I don't care since they will just give me another gift. I don't want it anyway.
Grandparents can sometimes get some outlandish gifts but know in their heart that they mean well. Even if you do not like the gift you should just say thank you and be polite.