Walls are not straight and sometimes gaps occur. This is why it's important to know what type of backsplash will be used, because many times a backsplash will cover the gap that is there because of the walls being so wonky. But, a 1/2" gap isn't up to standard. Perhaps they cut to the narrowest dimension rather than the widest and then cut into the drywall to install that pieces? Or perhaps they aren't a cut to size shop at all and use pre fabricated blanks that they only cut to length? There are many (bad) reasons that you would have 1/2" gap, but unless you still hold some of the $$ or are dealing with a very reputable fabricator (who wouldn't have had the gaps to begin with) then you don't have much pressure for them to make things right. I'd call them and set up an appointment for the supervisor or owner to look at your install in person and see where it goes from there.

1/2" is way too big. I would say 1/4" is too big. 1/8 might pass. You could fill 1/8" with caulk. If there is a back splash that covers it then as long as covered it is good.
They should have scribed to counter to match the wall.

This discrepancy should have been caught when the countertop guys templated the countertop.

Are your walls that messed up, or is it because of a flawed cabinet installation?

Regardless, a 1/2" gap is ridiculous.

You can have a new top properly fitted. Bingo, problem solved.

Is there a backsplash to be installed? That can hide a gap, with the exception of the gap at the end of the countertop. Depends on if that edge will be visible or not.

Everything else is a compromise of sorts in that you can reduce the gap in the back but you overhang in front.

You can slide it back but then your cabinet overhang at the front of the cabinets will be skewed.

You can scribe the countertop as is, slide the entire thing back 1/2" to close the gap on the right. That'll give you even reveal on the front overhang, but it'll be 1/2" less front cabinet overhang than you have now. Or you can do a fraction of that 1/2". Slide it back 1/4" for a 1/4" gap on the back left, and lose 1/4" of overhang at the front of the cabinets.

You can notch the drywall on the back right wall and slide the top back. Same effect as the previous recommendation, but you're notching the drywall instead of scribing the countertop.

Another alternative is to use mud to float out the back wall. Essentially you're making the back wall "thicker" to bring the wall forward to meet the countertop. It'd have to be done in several steps, and you'd have to feather the half-inch thickness down over the run of the wall. But that also depends on windows on the wall and how the existing window trim will be affected.

There are fixes and there are band-aid repairs.

Mongo