I've decided to put together a guide about stuff that is missing from GOG's versions of the Unreal games. Most of this probably applies to Steam and Anthology versions as well but I only have the GOG ones. If there are any errors in this guide, feel free to point that out. Most of the information comes from Liandri Archives.

Unreal Gold
What's included: Unreal Gold is patched to the latest official version 226.

Missing content: Fusion Map Pack ( / [url=https://www.dropbox.com/s/wxskfosxg10ugpb/fusionmappack.zip]Alternative link)
This is an official map pack that includes six maps (DM-Cybrosis, DM-Letting, DM-Loxi, DM-Mojo, DM-Shrapnel & DM-Twilight).
Bonus maps: Division Map Pack ( / [url=https://www.dropbox.com/s/hod3q9dntylhuog/division_ex_pr.zip]Alternative link / Info)
Three maps (DmBayC, DmCreek & DmKrazy) were released in various OEM versions of Unreal. Another three levels were included with the official strategy guide for Unreal (DmLocke, DmScruular & DmVilla2). The best way to get these maps is to download the semi-official Division Map Pack which also features 4 maps that were cut from Unreal/RtNP.

Unreal 2: The Awakening Special Edition
What's included: Unreal 2 is patched to the latest official version 2001. XMP is patched to the latest official version 7710.

Bonus maps: Solaris Base ( / [url=https://www.dropbox.com/s/vuaw4ogxzxgpzln/SolarisBase.zip]Alternative link)
Solaris Base is a map that was cut from the final game but was released by Legend Entertainment's level designer Matthias Worch (who also made a cool UT map called DM-MementoMori).
Other issues: XMP
GOG does not allow you to install Unreal 2 and XMP simultaneously.
Other issues: Intro
The intro is cut short on newer operating systems due to a bug. You can watch the full intro on YouTube.

Unreal Tournament GOTY
What's included: UT is patched to the latest official version 436 and includes the first three bonus packs.

Missing content: Bonus Pack 4 ( / [url=https://www.dropbox.com/s/ws9mcm9vu42uets/BonusPack4-no-umod.zip]Alternative link)
BP4 includes two models (WarBoss & Xan - Mark II), two texture packs and 12 maps (CTF-Beatitude, CTF-EpicBoy, CTF-Face-SE, CTF-Ratchet, DM-Bishop, DM-Closer, DM-Grit-TOURNEY, DM-Viridian-TOURNEY, DOM-Bullet, DOM-Cidom, DOM-Lament][, DOM-WolfsBay).
Missing content: S3TC textures (Download)
The second CD of the retail GOTY Edition included high-resolution S3TC textures. These introduced a few bugs like players skating in multiplayer and messed up skyboxes. There's a guide on how to fix these issues here but it's easier to use community textures and not bother with Epic's ones. Nevertheless, I'm including this here for completeness' sake.
Other issues: UMOD files
The GOG version doesn't register the UMOD file type association which makes installing mods harder. There are various ways to fix this. One possible solution is to use the UMod Browser program.
Other patches: IpDrv.dll ( / [url=https://www.dropbox.com/s/s8cgs14kex9hth8/ut436-ipdrv-070902.zip]Alternative link)
Updated IpDrv.dll for servers; fixes a denial of service attack and a Windows 2000 ping problem. I'm not quite sure if this is necessary. The one included with GOG's version is smaller but has a newer access date but so do almost all of the files.
Other patches: OpenGLDrv.dll ( / [url=https://www.dropbox.com/s/aq1nm5t9su9pukt/ut436-opengldrv-090602.zip]Alternative link)
Epic's beta OpenGLDrv.dll update. This is newer and better than the one included by default but I recommend UTGLR 3.7 instead.
Other patches: EAX (Main patch + + [url=http://www.gamefront.com/files/898908/]DM / Main patch + + [url=https://www.dropbox.com/s/1vne7inljo92eba/uteaxdm.zip]DM)
Creative's EAX patches. I'm not sure if they actually make the game sound better. First install the main patch and then the enhanced levels.
Bundled mods: Rocket Arena & ChaosUT (Rocket Arena + ChaosUT)
The second CD of the retail GOTY Edition included two mods. Rocket Arena includes some maps by Epic's level designers while ChaosUT is purely a community mod. Be sure to grab the GOTY Map Pack for ChaosUT if you're planning to install it.

Unreal Tournament 2004 ECE
What's included: UT2004 is patched to the latest official version 3369 and includes the Editor's Choice Edition and Mega Pack bonus content.

Missing content: XP Levels ( / [url=https://www.dropbox.com/s/g29spizqyiifanj/UT2004-ONSBonusMapPack.zip]Alternative link)
This official bonus pack includes two new Onslaught maps (ONS-Aridoom & ONS-Ascendancy).
Bonus maps: AS-Confexia ( / [url=https://www.dropbox.com/s/l7bpxc6p0fcycla/AS-Confexia.zip]Alternative link)
A bonus map by Streamline Studios who made ONS-Torlan.
Bonus maps: DM-Forbidden ( / [url=https://www.dropbox.com/s/64taxs8rhhpqi2u/dm-forbidden.zip]Alternative link)
A bonus map by David Spalinski from Epic Games.
Bonus maps: ONS-Icarus ( / [url=https://www.dropbox.com/s/e0opz5406zj4hcf/ons-icarus.zip]Alternative link)
A bonus map by Chris "Plutonic" Blundell.
Bonus maps: CTF-DE-LavaGiant2 ( / [url=https://www.dropbox.com/s/lbjvod46hopxr1r/ctf-de-lavagiant2.zip]Alternative link)
This map was in UT2003 but was not included in UT2004. This version has been ported to UT2004 by a community mapper.
Other issues: CD-key
GOG's version of UT2004 comes with a default CD-key for everyone. If you want a unique CD-key, you must contact support.
Other issues: UT4MOD files
The GOG version doesn't register the UT4MOD file type association which makes installing mods harder. There are various ways to fix this. One possible solution is to use the UMod Browser program.
Other patches: 64-bit patch ( / [url=https://www.dropbox.com/s/ul2m1njyh2iaeru/ut2004-win64-3369.zip]Alternative link)
This patch will give you a 64-bit executable for UT2004. It uses experimental drivers and is not officially supported by Epic.
Other patches: X-Fi patch (Download)
Another patch for Creative sound cards.
Bundled mods: ECE
The retail version of UT2004: ECE included 11 community mods. These were: Air Buccaneers, Alien Swarm, ChaosUT2: Evolution, Clone Bandits, Deathball, Domain 2049, Frag.Ops, Jailbreak, Red Orchestra, Rocketeer, UnWheel. If you're interested in these mods, use Google to find them. :)

Hi...

Feel free to give this a try:

files.ego-creations.de/patches/UnrealGold_Fix_GOG.exe

This unofficial patch for Unreal Gold adds missing files, file associations, UMOD support and fix the buggy UEd.
Only for the original release 226 not 227 ff

I'm author of many Unreal related tools @ ego-creations.de
Send me your feedback via support formular @ ego-creations.de

Great post dude!

Yeah, nice summary for the official extra content. Thanks for it.

For some reason, I'm not able to download any content for UT2004. Maybe there are problems with Gamefront servers?

For UT99 you should get all of the bonus packs, just to make sure they are updated to the latest versions. Also there is a patch for bonus pack 4.

http://www.uttexture.com/UT/Website/Downloads/BonusPacks/BonusPacks.htm

FYI the Capture the Flag map Hall of Giants is updatei to actually be a capture the flag map in the latest version of it's applicable bonus pack. If you like playing death match on that map rename the old Hall of giants map file, replace CTF with DM. Now you can keep that map for death match and it will appear in the death match map list.

I think GameFront banned residents of certain countries from downloading files from their service. I should add alternative links for the files when I have the time. You shouldn't reinstall the first three bonus packs. The files you linked to were released before the GOTY Edition and have been released only once so there's no such things as "latest versions" of them. In fact, reinstalling BP2 will give you errors when trying to connect to certain servers because the de.u file will be different.
Also Hall of Giants from the GOG version and from the bonus pack are 100% identical. The patch for BP4 is unofficial and I think it adds skin support for the two new models.

S3TC Texture Pack was awesome but you can't play online while using it (three years ago, tried that).

I wonder why those mods are not included in the GOG.com release of the Editor's Choice Edition.

I still have a physical copy of the game's original retail release (I bought the GOG version for convenience). I should be able to use the CD-key from that, correct? Possibly the same reason PC gaming magazines dropped the coverdisc as they considered that internet speeds were on the rise and it was easier for people to just download them.

[HKEY_LOCAL_MACHINE\SOFTWARE\Unreal Technology\Installed Apps\UT2004]
"CDKey"="XXXXX-XXXXX-XXXXX-XXXXX"

Replace XXXXX-XXXXX-XXXXX-XXXXX with your serial.

All the links are dead. Can you post updated ones? Reviving a dead thread, I know, yadayada

Replace every "www.gamefront.com" with "gamefront.online" (without www).