Yes, I agree with everyone else.
It’s not about giving a gift. When my father died - some people did give small amounts of money - $5 to $20 in a card. Some sent flowers. But most of all - what really is valuable is to be there for your grandma and your parents.
Help if you can - when you can - but i think just relieving some of the loneliness and grief (well, you can’t really relieve the grief, they will have to go through it). But having someone remember the individual is wonderful - and not just at the funeral - because there is so much to do that week - those people often don’t really feel the loss right away. But the weeks and months that follow - when people are no longer calling or visiting- that is when the loneliness can really set in.
Often people have no idea what to say - so they say nothing. Then because they have said nothing, they feel guilty about not having done the right thing - so the avoid the person who had a loss. There is no “right thing” to say - in fact, there is nothing that you can say that will make it all better. So once, you realize that, you can decide to be there for comfort and support - and for companionship. You don’t have to be there all the time - but just make extra efforts to have calls or visits - short ones if you want. And don’t be afraid to bring up the memory of the deceased. Don’t think that you might “remind them of their loss and they might start crying”. Yes, they might cry - but it wasn’t because of you - and it may be a good thing that they are crying to remember all that they had.
So when you say - "I will always miss how Grandpa used to … - that always made me laugh. or “It makes me sad that Grandpa wasn’t here today, because I heard this song and it reminded me of him.” Any story or thought you have is very good.
And - I am sorry for your loss.