I have opted in for the FREE Deus Ex revision mod. How can I remove this mod? It seems to self-install when installing the game on GoG Galaxy 2.0.

I suppose you're using GOG Galaxy (the installer downloaded via GOGDownloader installs Deus Ex without Revision).
In GOG Galaxy, select Deus Ex GOTY, go to the game's Parameters page and deselect the Revision mod (it's (un)installed as a DLC). The game will "update" by removing the Revision, letting you only the original game.

For your information, all of this is not necessary to play without Revision ; the original game can be launched separately. At the installation, you should have two launchers : one for Deus Ex GOTY and another for Deus Ex Revision. It can be explained by the fact Revision does not overwrite the original files, but simply put itself in a Revision directory with its own resources.

Hope I helped you.

This same for many other games too, according to complaints. If sufficiently many report the bug they might fix something as important as that before 2030. Please contact GOG support.

You can opt to not install it or remove it after installation by clicking the two solid lines button by "Play" and >manage installation and then to configure and remove the tick by deus ex revision.