I'm resentful of my grandparents for not inviting my gf to their anniversary meal
What is your reason for being resentful to your grandparents?
They make really good chicken balls and I wish she could have tried them.
It is rude to complain about someone else's plans. If you want to have her taste them you can ask for the recipe and make them.
They don't have to let her know if they don't want to. I can't afford to make them.
It is up to your grandparents if they want to invite your girlfriend. If you really want them to involve your partner, you should talk to them about this. Maybe they will hear you out and decide to include them in their plans.