Welcome, Grant.
I trust you're actually wanting to make your walls flat, rather than level. If you make them level, they'll be part of your floor, eh?
I've never worked with that GoBoard. Never even seen a piece, truth be known, but it seems to be more available than the earlier brands and we see a lot of it here on the forums. That said, since it's apparently designed to be bonded to with cementitious products, I'd guess you could flatten it using any decent cementitious patching material. That is not a recommended use for thinset mortar, but people use it for that purpose anyway.
Far better would be to correct your wall framing such that your wallboard is flat when attached.
The tile industry standard for flatness of the substrate is no deviation from intended plane of more than 1/4" in ten feet nor 1/16th" in one foot. That would be sufficient for the tiles you've selected, but flatter is always better.
My opinion; worth price charged.