Armin has uploaded an installer that contains the 3rd Edition Books data files that were converted over for GCA4 some time ago. If you have GCA 3, you already have these files. If you have GCA 4, you can now get them.
You can download it here:
3rd_edition_books.exe
Note that there is no guarantee that all of these files will work for you as you might expect. They haven't been extensively tested. Also, if you want to use them with GURPS 4th Edition, you must be aware that the skill types in these files are for 3rd Edition, so they'll need to be changed to work with 4th Edition.
It's worth noting that much of the functionality that is available for 4th Edition data files in unavailable for the 3rd Edition files. Interest in 3rd Edition support for GCA has been almost non-existent, so these files have not been maintained and updated.
Also worth mentioning is that "Powers" in 3rd Edition are placed under the "Perks" tab within GCA4. This is because in 3rd Edition "Powers" were treated as a separate class of traits, and there were no "Perks," so the only place to separate them out was into the "Perks" tab.