Sliding Gate Mis-Aligned After Repairs

If the gap was good before he cut out the bad part, there isnt much you can do......A inch opening is a lot, you may be able to adjust the rollers and reduce that but it will never be perfect
When a repair like this is done, the gate has to secured in such a way that when it is released (cut) it will hold square
It's the same principle as when a section of a car frame is cut out.
It doesn't take much if when the gate was cut for the thing to pull
Id run a piece of flat stock, 10 gauge, 1 1/2 or 2" wide plum top to bottom, tack it, round the edges, paint it to match, do that for both sides and call it a day