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.