I personally used an OpenSUSE box for all my Ruby/Python and Java dev and never looked back... its rock solid and i was able to just use CLI or load up Gnome/KDE etc on the fy to test my apps/interface.

OpenSUSE is the absolute best way to go which affords the most dynamic and fluid movement for testing.

I'm gonna put in another vote for Arch Linux. By far my favorite distro for anything.

So when it comes to Linux, does GOG support plain Debian? Or do they only support Ubuntu and Linux Mint?

GOG only officially support Ubuntu and Mint. But you can be fairly certain that the game runs on many other distributions as well (I use Arch and have yet to have a problem).

"Supports" and "runs on" mean very different things.

So if I want to play games on Linux + develop code too it will be easier for me to use Ubuntu / Mint and run Visual Studio on some sort of Windows Virtual Machine?

Probably, yes.

I have frequently had issues where GOG's launcher script (which sets up an environment with their bundled libraries and whatnot) doesn't work - probably since I'm most likely running newer versions of said libraries - so I've had to run the games' executables directly.

OP, take a look at the different Fedora Linux spins & lab builds.
https://spins.fedoraproject.org/
https://labs.fedoraproject.org/

Fedora works pretty great, and is really similar to RHE.
Fedora uses more community managed software sources than RHE, and is the test-bed for new code that eventually gets stress-tested in Red Hat Enterprise QA servers.
I really prefer how Fedora's security model works over the Ubuntu method.

RIght now, my main system is running Fedora Labs Games.
XFCE is the default Desktop on FL Games variant, installing the KDE Plasma desktop was painless.
You may run into a annoying KWalletmigration agent pop-up in KDE that will not go away.
This procedure makes that popup go away.
https://forums.opensuse.org/showthread.php/510917-New-leap-final-installation-on-laptop-On-each-login-get-twice-Kwalletmigration-agent-asking-for-pa

Can you elaborate on that ?
Btw it seems Debian also have something like those - its called Pure Blends (obviously its a trend), but I like the presentation of what's inside of Fedora ones, better.

One more thing leon30,

I found a guide for installing IntelliJ IDE on Fedora 23
https://technoref.wordpress.com/2016/01/19/install-idea-intellij-in-fedora-23/

You may or may not have to install additional development tool packages.
See below for details

==============================
Fedora does not include the same "build-essentials" package that Ubuntu distros use.
If you plan on recompiling software or doing any kind of coding work, you will need to install additional software.

Whatever variant of Fedora you install will use yum or dnf. dnf is the next-gen replacement for yum, yum & dnf are functionally identical, so the same commands will work on both.

miminalist coding software for Fedora:
open a terminal window, su - to root , or sudo the following:

dnf install make automake gcc gcc-c++ kernel-devel
Ex: "sudo dnf install make automake gcc gcc-c++ kernel-devel"

For installing full blown development tools & development libraries in Fedora,
open a terminal window, su - to root , or sudo the following:

dnf groupinstall "Development Tools" "Development Libraries"
Ex: 'sudo dnf groupinstall "Development Tools" "Development Libraries" '
...without the ' comments

===========

The official Ubuntu wiki explains this better than I ever could.
https://help.ubuntu.com/community/RootSudo

tldr version:
The Root id & root id usage is confusing for a new to linux user.
Therefore, Ubuntu hides/disables the root id,
and makes every user id in Ubuntu able to use the sudo command.

This is Good and Bad at the same time.