Tested.

All countries are 10k acres, 1.15m nw
Country A GDI
Country B NON GDI
Country C GDI

Country A hits Country B : 864 acres
Country A hits Country C : 864 acres


All countries are 10k acres, 1.15m nw
Country D GDI
Country E NON GDI
Country F GDI

Country D hits Country E : 958 acres
Country D hits Country F : 958 acres


Conclusion: Countries in GDI countries have a -10% gains when attacking someone(be it GDI or non-GDI).

Wow , I didnt know that. lol
Even if you knew this, are you not going to join GDI?