I'm sorry to hear about your grandfather.

Most sushi places will organise a catering tray of cut up sushi which is popular, easy etc.
And I'd do a big cheese platter (buy big chucks of cheese, put out a few knives, with a selection of crackers.

I'd assume that people who bring things, will probably bring sweet stuff. So I would focus on providing savoury options.

But don't stress too much about it - no one is going to complain about it! I think that food is just a reason to stay around and chat and remember and grieve together.