There is a map editor accessed via the main menu. There is no online/offline multiplayer.

Does it mean that they have finally added a new launcher that has been available on Steam since 2018?

I haven't downloaded and tried it out yet, but that's what it should be, yes.

I have tried it but I'm not sure it is the same version. There is no launcher and it seems to be just the original game with the Advent Revising patch included, but I don't have the Steam version so I can't really compare if they are the same.

What does the patch do?

Advent Revising v2.00

Advent Revising is an unofficial fan patch designed to improve the cinematics of
Advent Rising, restore removed cutscenes, and fix various other errors in the
game.

This English patch does not fix gameplay issues, but corrects common problems
with movie playback such as audio desync, low framerate, and inaccurate
lip-syncing, as well as replacing the content that was cut in the PC version of
the game. Additionally, in-game subtitles have been completely redone,
rewritten, and retimed.

This is the full version of the patch that aims to address even minor annoyances
in the game's cutscenes.

************************************************
Features
************************************************
- 12 Xbox cutscenes and one movie file that were removed from the PC version
- Fixes to common movie playback issues, such as audio desync and low framerate
- Edits to 83 movie files and over two hours of video content
- Corrections to timing and subtitling errors in all of the game's subtitles

- Edits to the .ini files (available in separate download) to:
- Enable full 16:9 display and remove letterboxing
- Enable trilinear filtering and Vsync
- Enable partial Xbox One controller support
- Bind V to teleport, F5 to ShowDebug, and F6 to ViewBot
- Bind F7 to free camera, F8 to level skip, and F9 to screenshot
- Edits to the game's engine.int to correct a few problems with objectives
- Edits to the game's Interface.int, EonWeapons.int, and EonPowers.int to
correct some issues with weapons, weapon screens, and descriptions

v2.00 additions (Full version)
- Higher quality Xbox-exclusive cutscenes
- All cutscenes re-edited, with visual corrections
- 15 cutscenes new to patch v2.00
- 7 newly edited movie files

The versions are not exactly the same. While PCGamingWiki mentions that Advent Rising on Steam also uses the Advent Revising 2.0 patch, the GOG version doesn't have the launcher (see images below). While those options could potentially be modified via *.inis on the GOG version, it doesn't change the fact that it's less convenient to do so.

I guess GOG decided not to include this launcher because it has some problems with the decimal separator.

I remember Steam users figured out that this launcher doesn't work on operating systems that use a comma (instead of a period) as the decimal separator.

For example, a comma is used as the default decimal separator in the Russian version of Windows, where this launcher refuses to work.

I don't know if it's possible to fix this issue simply by changing the decimal separator in Windows settings (instead of completely changing the OS language), but I can install my Steam version later and check it out. (:

If the trick works, I think we can share this information with GOG and ask them to add the launcher at least as a secondary way to launch the game, because it's more convenient than manually editing the config files.

I take it the Steam version is DRM Free?

Last time I checked, it wouldn't run without the Steam client.

I was wondering what's behind the "Controller" section in the Steam launcher since I have some troubles configuring my controller with the GOG version.

Yep, changing the decimal separator (from comma to period) in the system settings is enough for the launcher to work.

I also found out that the default decimal separator in the Polish version of Windows is also the comma. I guess the GOG team was unable to start the launcher because of it and decided not to include it. If it was the only issue with the launcher, it has now been solved. (:

The "Controller" section contains the XInput controller settings. Unfortunately, controller is partially supported: it's not supported in the menus, and triggers are not registered by the game.

Err.. changing the system decimal separator is kinda terrible though (for example if you use Excel), compared to editing a config file of a game.

How exactly can this change break Excel?

If it's really something serious, then yes, it's better to tweak the config files manually.