I wonder if anyone can help me out, all my full screen DOSbox games have a 1 pixel red line on the right-hand side of every game. It doesn't matter if I go windowed and then full screen again.

MacOS 10.14.6

nvm, I see the question was way too hard to answer.

I have GoG Galaxy 1, do you mean to say if I update to GoG Galaxy 2 it should be OK?

I think the executable I was using might have been meant for GoGG 2 since I just copied over a much older copy of the same game from my laptop, and not only is the red line not there, but the GoG wrapper is also working properly.

OK. Do I have to do anything to make it update?

Report the problem to GOG support and link to this thread. They will see the problem and the solution in a glance.

While you are trying to get GOG response you may download latest DOSBox for Mac and replace DOSBox files for your game to see if it solve the "red line" issue.
https://www.dosbox.com/download.php?main=1

Figured out how, and it works, but I still have the issue of the GoG GUI overlay not being used.

As far as I know GOG overlay doesn't work for DOS games under DOSBox.

Not true, look. This has always worked for me on all DOS games in the past.

https://imgur.com/jqCUKeq

edit: with the red line thing solved I ought to make a new thread about the overlay thing

I didn't know that. It's odd how simple DOSBox files replacement could ruin Galaxy functionality. Perhaps you can try to reinstall the game (or repair it to older version) to revert back to overlay functionality. And then copy just 1 file (DOSBox executable) to your game folder to see if it works, if yes then add remaining files one by one.

No, this was already broken BEFORE I did any fiddling.

If it was not there in the first place and you have solid evidence that overlay works for your other DOSBox based games than it's a different story for tech support to tell.