What is Max's relationship with Gram like? Include textual evidence to support your answer.

Whether we are asking about Gram or Grim, the relationship with Max doesn't change. Gram and Grim are Max's grandparents, and they love him very much. Unfortunately, there is a bit of a barrier that exists between them and Max. Part of that barrier is Max's fault. He intentionally tries to distance himself from them. He does this because they remind him of his mother.

Gram and Grim, bless their pointed little heads, they're my mother's people, her parents.

Normally, that would be a good thing. However, Max has some serious emotional baggage in his life because of his mother. He saw her murdered by his father. That event is the other part of the barrier that exists between Max and his grandparents. They love Max a lot, but he looks like his father. Max is a daily reminder to them of what happened to their daughter.
Despite this barrier, Gram and Grim do love Max and put their love into action. We are told in chapter 2 that they built the special "down under" space just for Max.

in the little room Grim built for me. Glued up this cheap paneling, right?

This down under space gives Max the room that he needs and wants. Gram and Grim might not be always interacting with Max, but that doesn't mean they don't love him and teach him to be a good person. Their constant love and guidance is felt throughout the story, and it can be seen in how good of a person Max is. Max might act aloof about his grandparents, but he does listen to them, respect them, love them back, and apply their life lessons to his own life and friendships.

It all goes to show, like Grim always says, that you can't always judge a book by the cover.