Read the ingame description of GDI, it is different in Primary.
You can only join GDI if you haven't attacked or done a harmful spy op.
There is no 'GDI range', instead a country in GDI is protected from special attacks and harmful spy ops.
There is a humanitarian range of four times bigger/smaller.
see Details for Primary here:
http://www.earthempires.com/primary/serverdetails
I suspect you are prevented from retaliating because you are trying to lob a missile or AB your attacker, or he is outside the humanitarian range. Only if that country in GDI attacks you more than once will you be able to retal with GS, BR AB, missiles or harmful ops.