-mapedit switch (or -mapedit my_editor.so/my_editor.dll)

[NickDoom]

Instead of running the map, the engine loads the map in "tab map" mode. The gameplay itself is frozen, but you can explore the level in 3D mode (both physics-checking and noclip). If you change the geometry in the "tab map mode", the engine calls a reject builder and a node builder before entering 3D mode.
Yes, I'm speaking about an in-built map editor (or an interface to a editor library, but it must be as cross-platform as the (G)ZDoom itself is).
WADED-era editor is incredibly simple and takes few Kb (if you have the "tab map", you already have 99% of this editor) but having auto-editing options such as "curve linedefs" etc can turn (G)Zdoom into a bloatware. So maybe it's a good idea to attach an external edit engine.

[Enjay]

Honestly, other than seeing exactly how GZDoom would render the map, it sounds like what you are asking for is covered very nicely (and then some) by Ultimate Doom Builder. And if you need to see things in the actual game, if you've set up UDB properly with testing options, hit F9 and you're in the game a few seconds later.

For what it's worth, DeePsea's 3D map editor is a modified version of the Risen3D source port. So it does indeed use a game engine for in-map editing. DeePSea is pretty out of date though (especially as it does not support UDMF).