Exporting to Garrys Mod (GMod) - SWTOR-Slicers/WikiPedia GitHub Wiki
At the moment we don't have proper guides for this (we'd welcome anyone wanting to give it a go). We'd suggest giving a look at our guides on baking and on SWTOR's materials and textures system.
Those aside, here there are a few references that might be of help:
VALVE Developer Community: Source and the Source SDK (GMod-related information at the bottom of the page).