That is a fan made mod. On Mod DB you can find it alongside with other cool mods.

Mod DB

Thanks, I know that it is a fan made mod.
But the adittion to the physical release of the GOTY edition was a second cd that had Chaos UT and few more bonuses on it. I find it very weird that it is not included on GOG's version (it's basicly the "vanilla" version of the game).
Adding to this, the manual is reffering to CUT as it is included in the game (GOG's manual)

Oh I see. The same was with the UT2004 too. There was mods included with the DVD release. I think the GOG was not able to connect with the owner of the mod, this is why they not included anything other than the vanilla game. But at least these mods still exist on other sites.

Well... Thats crap.

I know that those mods are free and available online but they are what made the GOTY edition diffrent from the original. It's very strange that GOG's version isn't 1:1 to the physical version.

Yeah, I completely agree. At least they could add these once included things next to the game's installer. A GOG version of the original mod.