Pay closer attention to the descriptions of workshop items when you have issues. It explicitly says that you will not find the Doom maps in the normal map list, because that's not how this addon works. Most workshop maps will give you Source Engine BSP files to load, and will show in the map list, but this addon loads Doom maps directly from the WAD. Garry's Mod can't deal with this directly, so it works by loading the maps within the space of another map. Load up flatgrass, and check the spawn menu. There will be a Doom Maps tab near your toolgun tools. One of the screenshots shows this.

Ah.