GZDoom Builder will automatically load data from GLDEFS and MODELDEF lumps if they are present in map resources.
MODELDEF support: all relevant MODELDEF properties and flags are supported. Sprite translations are not supported.
GLDEFS support:
only dynamic light definitions are currently supported. Brightmaps and glowing flats support may be added in the future. You can use "//$GZDB_SKIP" special comment to abort parsing of the current file.
You can reload GLDEFS and MODELDEF by using "Reload GLDEFS" and "Reload MODELDEF" actions.
To enable GZDoom's built-in GLDEFS dynamic light definitions, you need to add "lights.pk3" as a map resource. Don't forget to check "Exclude from testing parameters" chekbox while adding it, otherwise GZDoom won't load your map. "lights.pk3" can be found in GZDoom folder.
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 a thing sprite, or to attach a light defined in GLDEFS, GZDB 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'll need to add a new value named "class" to thing definition in game configuration:
3001
{
title = "Imp";
sprite = "TROOA2A8";
class = "DoomImp"; // <- you'll need to add this value
}