Aside from installing maybe some shared libraries, i can't think of a reason.

So after ~2 hours of investigation, getting into an argument with my wife because it's weekend and I should not be working, skipping a breakfast and drinking two coffees I found out that the admin rights are not necessary at all.

For people that would rather stay safe and not give root to every script downloaded from the web, here is a short instruction how to extract and use the .app from the .pkg bundle. (procedure might vary from game to game, but you can use it as a general outline)

Open terminal
in the terminal type
pkgutil --extract /path-to-your-pkg /path-to-the-folder-to-extract

navigate to the folder you have extracted it to.
If you see another pkg there, try right-click on it, and select "Show Package Contents"
If you can open it, then go "Scripts->payload". In the payload folder there should be "Contents" folder.
Copy (cmd-c) that folder.

Navigate somewhere else (e.g your Downloads folder).

Make a folder named "name-of-the-game.app" (without the quotes). if you use Finder it will complain, but just click "add".
Again, right-click on the newly created .app folder and select "show package contents". Once you are there paste (cmd-v) the "Contents" folder.

Now if you navigate out of the .app folder you should notice that the folder magically changed the icon to the game icon. Now it should be a fully functional Mac app.

Move it wherever you want and enjoy playing your game without any admin rights necessary.

and again, shame on you GOG!

What games require admin privileges? Shared libraries should be contained in the app bundle. All the games I have played were just drag&drop apps.

EDIT: DAMMIT

It's an installer for Halcyon 6. But as I wrote above - the game itself works fine after manual extraction, it is just GOG installer that asks for admin password.

That's the odd thing: Mac apps should not be having an installer in the first place. Any libraries the game needs are either installed on the system by default or included inside the app bundle. If an installer needs admin privileges without very good reason I consider that to be defective by design. You should contact GOG support or the developers to figure out why on earth the game needs an installer.

there is no reason. before pkg there was just the usual .dmg with a .app inside.
also the current pkg is a really stupid mess, even when you extract the game inside an external drive the installer try to use the internal disk (and if you have a small ssd that's a real problem).

it is just a stupid change... some times ago all the .dmg were updated to .pkg for no reason.

Unspecified "internal technical reasons".

fixed.
since they and lots of developer before offered dmg, there is no reason to use pkg at all.... just take a look to almost all applications you can find for mac... there is almost no pkg.

No. Try using generic quotes there, without _#.

Didn't pursue it, but my impression was it was something gatekeeper related they were attempting to workaround.

I don't know what are you talking about with the use of _#.... I didn't use that... maybe it is something that I got because of gog essentials.
Gatekeeper is a user business, if I want to install games outside the mac app store/from developer that have not paid 99 dollars, I can allow it, temporaly by open it with alt or in a permanent way by use the system preferences.
With a pkg when I need to extract a game, I need to have enough GB of free space on the internal disk, because there is no way to extract it on an external drive... and I need root permission, or the teminal. This is much worse than the Gatekeeper problem.

No, you did. It's the default when clicking reply to a particular post. I wasn't planning on wasting two responses about this, but I should have been more verbose the first time. I mildly dislike being deliberately misquoted. If you feel the need then change it into a generic quote block by removing the _# (e.g. _9) from the initial quote tag.

It's just installing into the system-wide Applications directory. If you had a .dmg file you'd just drag and drop it and the system would request permissions, instead of the installer doing so.

Thank you man!! Exaclty!

I gree with you. I though GOG guys were the good guys...