On the main page, it says it works at all default FSX airports but nothing about freeware.
From the GSX page:
http://www.fsdreamteam.com/products_gsx.html"Works with every FSX airport, both default and 3rd party"Doesn't say anything about 3rd party = payware so yes, freeware will obviously work, provided they come with a complete AFCAD without errors.
SOME freeware airports are just enhancements built on top of the default AFCAD, and this method is not supported by GSX, which will use only the data from the default AFCAD. Basically, the AFCAD that comes with the scenery must be self-contained, and this is the most common case.