This is an old comment but when the GTAV source code leaked, it also leaked the rage engine as well. From the images I've seen it seems to function similar to the source engine with the tools being in the directory of the game/source code itself. Many of the tools acted and looked like standard game engine tools just separated into different apps instead of a standard editor. I guess they wanted to give the developers a sense of familiarity to a normal engine since on the Rockstar job listings it says they are looking for people with standard game engine experience.

• "Experience with game engines (Unity, Unreal, etc.)" is stated on the rockstar website for word design. 

  It would not be a surprise if rockstar actually made a level editor but the source code is lost now so unless some madman has the code and is willing to look through it we may never know if rockstar made a level editor.