I wanted to add a few abandonware DOS games to GOG Galaxy, but there's an issue. When I link the executable to the game's shortcut which itself links to DOSBox but configured to mount and run the game automatically from the shortcut arguments, GOG doesn't run the shortcut with the autoexec config, but instead just runs DOSBox normally. And when I try "Manage installation, Configure" in GOG Galaxy to try adding the shortcut arguments directly to GOG Galaxy, it's greyed out and I can't do it.

So, does this means DOSBox games is no good for GOG Galaxy since it only works with actual executables, not shortcuts, which you need to do shortcuts to run DOSBOX games smoothly to skip the boring commands every time you run the game? That's a bummer.

This is a game shop. Discussing pirated games here is not a good idea.

It is possible to do what you want. Not telling more here but here is a link to a useful website for your DOS troubles:
https://www.vogons.org

Is it piracy if the game is abandonware and isn't sold anywhere in the world? The game in question I'm trying to add is Birthright - The Gorgon's Alliance, a game I can't buy anywhere. Either way, thanks for the link, I'll take a look at it.

Legally, yes, unless a game has slipped into public domain, which really does not happen very often.
The decision whether or not to keep a game available is a business decision, has nothing to do with the rights to a game. IThe copyright owner retains the rights regardless whether or not they keep the game in print or not. Same rule applies to books, movies etc.
It sort of a dead letter law; since most abadonware games owners thing the legal costs of making a site take down the game is not worth it since they think the game is pretty much dead anyway, but, yes, from a pure legal point of view downloading a abandonware game is piracy.
GOG does not want to support piracy in any form since it could create major legal problems for them, and lawyers are expensive.

Basic rule for any business is to avoid anything which might end up in court; legal costs can kill you.

I get it, no talk of piracy. Either way, that isn't really related to my question. There's an option to manually add games, it's a feature of GOG Galaxy. Still, that feature doesn't seem to work fully, at least with shortcuts, and the option to configure the shortcut arguments is greyed out.

In some countries, yes. And since GOG does business in those countries, they don't typically allow the kind of discussion. In the US , for example, the only legal concept related to abandonware is public domain, and there are not any games old enough to fall into that category.

Yes. Just because a game isn’t sold anymore doesn’t mean no one owns the rights to it. Copyright can only end in three ways: it expires (usually 70 years after publication), it is released into the public domain, or it’s transferred to someone else. If a game is simply ‘abandoned’, meaning it’s no longer sold, that still means someone still owns the copyright. So, legally speaking, abandonware counts as piracy.

(You can certainly debate the morality of downloading and using abandonware, and an argument can be made that it is important for software preservation, but this is one of those cases where law and ethics might not line up.)