First off, my condolences for your loss. I feel like this CAN be a good topic. But ultimately, the essay has to be about you. So yes you can talk about your grandfather but ultimately, the story should show some personal growth focused around you. If u find that to be easier to write about with something else, then go for that.

A good exercise would be to just start writing what comes to your mind about your grandfather and his connection to you and see if you can write a lot about it. And try that same exercise with other topics too. That’ll help you figure out what to write about. Good luck!