And if you are talking about entities, (which are the things in a map that tell the game where a door is and how it moves, and how the transporter in RPG-X EF works etc), you need to do an extra thing, because GTK needs to know what entities are supported by RPG-X. At the time of posting this i dont have time to explain it in detail, but basically there are 2 things:
-Brushes/Patches: the things you actually see, like the physical warp core, the door, etc. Everything that renders in a game is either a brush or a patch.
-Entities: The things that you don't see but are there to make stuff happen, like doors, lights etc.
Furthermore, have a look at what Klaw posted, thats good advice

Just use radiant and see if you can make a first easy map. Start out with just a box as a room and go from there. If you have any specific question after that, feel free to post them. A general "how does gtk work" question is pretty hard to answer.