[ericbsmith]

1. GURPS Basic Set 4th Ed.--Characters.gdf must be first (unless you're using GURPS Lite 4th Ed.gdf instead)
   
2. After that you can load any non-worldbook data files (the files in the base folder) in any order you desire, with a couple of specific exceptions the order really doesn't matter.
   • GURPS Power-Ups 3 Talents - Alt Cost.gdf requires that GURPS Power-Ups 3 Talents.gdf be loaded before it.
   • GURPS Power-Ups 2 Perks 4e - Charms.gdf and GURPS Fantasy 4e - Fixed Magic.gdf require that GURPS Magic 4e.gdf be loaded before them.
   • GURPS Supers 4e.gdf requires GURPS Powers 4e.gdf
   • GURPS Martial Arts 4e - Gladiators.gdf requires that GURPS Martial Arts 4e.gdf be loaded before it.
3. Next you will want to load any worldbook files you will be using, and if there are more than one worldbook file for the gameworld you are setting up you should load them in increasing numeric order (that is, if you are loading a Dungeon Fantasy game you should load GURPS Dungeon Fantasy 1 - Adventurers.gdf first, then you'd load GURPS Dungeon Fantasy 2 - Dungeons.gdf, etc.). Then you can load any miscellaneous files for the worldbook, such as the GURPS Dungeon Fantasy Pyramid.gdf file.Last you'd load the the Remove Unnecessary Traits for the worldbooks, assuming there is one (e.g. you'd load GURPS Dungeon Fantasy Remove Unnecessary Traits.gdf after all the other Dungeon Fantasy files).
   NOTE: Worldbook files are oftentimes destructive; that is, they make campaign level changes to the way that traits work. You should only load data files from ONE gameworld at a time to avoid errors and conflicts.
   
4. Next you'd load any homemade or downloaded non-official data files.
   
5. Last you'd load any Variant data files - those data files found in the Variants folder. Again, order doesn't really matter here, although you will want to load Variants which make changes before you load destructive Variants. For example, if you are loading a Variant file which changes Equipment to Metric you will want to load that before you load the file that removes Equipment above TL 4. The reason for this is more pragmatic than necessity; GCA unfortunately slows down loading dramatically if it is trying to make changes to traits that are no longer present in memory, so if you remove a whole lot of traits or equipment then try to make changes to those same traits and equipment the load times will balloon for no real gain.

Note that the data files have an ordering built into them; they know which files must be loaded before them in the data set order, and will pitch an error if you try to load them out of the expected order.