[Unreal Tournament GOTY]
Why doesn't this release come with the advertised GOTY stuff included, eg Chaos UT?

Rocket Arena and ChaosUT are fan-made mods, so there might've been some trouble with the rights. Apparently, the Steam version is missing them too.

You can download them manually:

https://www.moddb.com/mods/rocket-arenaut

https://www.moddb.com/mods/chaosut

I was under the impression that GoG supplied the 2 CD version.
Perhaps CD 2 counts as a DLC ?

A few years ago I made a list with all the missing things, what they are and what you can ignore.
https://ut99.org/viewtopic.php?f=6&t=12927

Better and bigger textures are now available, and the Chaos and Rocket Arena mods were updated after UT was released
so you will want the new versions anyway.
The Rocket Arena site no longer exists so ModDB is the best place for it.

However on behalf of the Chaotic Dreams team, we would prefer it if you get the files from us. We did not upload to ModDB so cannot vouch for the files.
https://chaoticdreams.org/chaosut/
(I added a bunch of bonus Chaos stuff I collected over the years in the thread at UT99.org)

Copyright issues is not why Chaos has not been supplied with the GoG release.
Nobody has asked us yet as far as I am aware.

Oh damn, nice to see such an old community still kicking!

You better believe it !
The original Quake 2 Chaos clan still exists and plays regular matches.