The mod usses the "Name" system, meaning that in order to spawn one of the GDF NPCs, you first have to spawn npc_citizen and then set their name to GDF xxxx.
Honestly I do not know how to do it from in game, howhever, by coding, Ihave come to this :

function replacement()
for k, v in pairs( ents.FindByClass( "npc_citizen" ) ) do

local chance = math.Rand(1,100)
if chance > 75 then
v:SetName("[GDF] Soldier")
end
end

end

It first discovers the npc_citizen, and then, by chance, it replaces them with the GDF