Linux support is something we want, but will have to wait until after we get a windows version going.
As for not being open source, its just what we know. We intend to release what source we can, so you will be able to mod it to your hearts content. The majority of the code is set up to be expandable, allowing you to easily add new equipment, ammunition etc.
For instance, adding a drum magazine for a AR-15 style weapon is as easy as creating an archtype for the 'GBMagazine' class, specificying a model, ammunition type, capacity and setting its attachment point to MAG_556STANAG. It will then work with any weapon that has its magazine attachment point set to MAG_556STANAG.
Most things are based on attachment points, which does allow for some sillyness.
eg. 5-6 ACOGS, bipod on an M9, scopes on side rails etc.
Gear queers wet dream

-Kris