Generally speaking, you cut the existing drywall back to the next stud so the edge splits the stud before applying new drywall of the size you are talking about.

Use your utility knife to cut the remaining drywall to a straight line. That leaves you with a straight sided area to measure.

You should have trimmed back and replaced the drywall BEFORE the granite went in. That would have given you a chance to make sure the wall was flat to match the granite with no gaps.

About the best you can do now is slice it straight along the edge of the counters.

But you're right, the tile will cover the less-than-ideal join, so don't sweat the small stuff. If it's still really messy after doing your best to trim it straight, use a piece of drywall mesh tape to cover the join. Paper tape is good too, as long as you get your gaps filled and covered. You're going for a functional flat surface to which you can adhere your tile, not a surface you will ever see again.

lazy is right, this was not your finest idea.

The only reason you're replacing the drywall is to close off the wall completely, not to give yourself a flat surface on which to tile - your backer board will give you that.

You are using backer board, right?

This means you are free to use thinner drywall in your replacement, tape and mud as needed, because you're going to make up the difference in HardieBacker (link below). Or, you could attach it directly to the studs and skip the drywall altogether.

You're tiling this yourself, aren't you. Google extensively first, and good luck!

Here is a link that might be useful: HardieBacker