Primary
GDI doesnt block you from getting hit.
If you are in GDI, you will be exposed to Special attacks (GS,BR,AB,Missles) if you double tap anyone.
If you are not in GDI, you are to exposed Special attacks (GS,BR,AB,Missles) to Non GDI players. You will only be exposed to Special attacks (GS,BR,AB,Missles) if you double tap someone who is in GDI
Humanitarians stops you if country is 1/4 or 4 times your size

Alliance
GDI blocks someone 1/2 times your size or 2 times your size from hitting you. Countries that are in GDI and is 1/2 your size or 2 times your size would have to declare war with your country before they can hit you or vice versa
If you are not in GDI - You are exposed to ALL attacks (GS,BR,AB,Missles) by anyone
IF you are in GDI - You are exposed to ALL attacks (GS,BR,AB,Missles) by anyone if either the attacker is 1/2 - 2 times your size or if they have declare war on you.
Humanitarians stops you if country is 1/12 or 12 times your size


Express
GDI Doesnt block you from getting hit
If you are in GDI, you will be exposed to Special attacks (GS,BR,AB,Missles) if you double tap anyone.
If you are not in GDI, you are to exposed Special attacks (GS,BR,AB,Missles) to Non GDI players. You will only be exposed to Special attacks (GS,BR,AB,Missles) if you double tap someone who is in GDI
Humanitarians stops you if country is 1/12 or 12 times your size


Tourney - Everyone is forced to join GDI. Countries that are 1/2 your size or 2 times your size would have to declare war with your country before they can hit you or vice versa
Humanitarians stops you if country is 1/8 or 8 times your size.


Team - Not sure
FFA - Not sure