Want to help my Grandfather and wanna know how?

Gregory, athritis is hard not only because of the pain but because of the things we have to give up. Eventually we learn to substitute other activities and that helps. Perhaps you can help your grandfather with this. Some people who can no longer cycle can still walk; others can swim. You might try these with your GF.

Because you're young, you naturally think of physical activities for bonding. If your grandfather can no longer do these without pain, try thinking of less active things. Does GF have a hobby or interest? Woodworking, cars, birds, chess or the like? These could offer you a chance to spend time together without causing him pain.

I'm sure spending time with you as he always has would help both his spirits and his pain. Good luck to both of you.