I haven't seen any convincing evidence of this yet. It's definitely a possibility, but we shouldn't act like it's been confirmed.

As I said in another comment, groypers and their ilk coopt existing symbols to identify themselves. They act in the realm of plausible deniability, making it very hard to prove who is and isn't part of the movement.

Most of the inscriptions could also support the conclusion that he's on the left. (More plausibly than they support the groyper idea, imo)

Or maybe he's just a nihilist who wanted to be famous and filled the bullets with ironic memes and video game references.

The case is not closed on his motivations.