If you are creating maps for Doom or Doom 2, you probably don't need to read this, since required information is already added for those games.
To load models or dynamic lights defined in GLDEFS for things defined in configuration files:
To display a model instead of thing sprite, or to attach a light defined in GLDEFS, my code needs to know a thing's class name (because that's how overrides are defined in MODELDEF and GLDEFS). Things defined in Doom Builder configuration files don't have this value. So, if you aren't using configs, which came with GZDoom Builder, or you are creating maps for games other than Doom and Doom 2, you need to add a new value named "class" to thing definition in config:
3001
{
title = "Imp";
sprite = "TROOA2A8";
class = "DoomImp"; // <- you need to add this value
}
You can find class names for things on ZDoom wiki
You can reload GLDEFS and MODELDEF by using "Reload GLDEFS" and "Reload MODELDEF" actions.