Detaching wires from an outlet doesn't seem to affect others on the circuit
You are going to have to identify every outlet on the circuit you want to place the GFCI on.  Just turn off the breaker and test with anything,  A simple lamp will do.   Most likely you'll find the head of circuit for your GFCI location. EDIT:  The wires in the outlet boxes aren't pig tailed are they?  You should have 2 blacks, 2 whites and grounds connected to each outlet.  If pig tailed, that would explain why other outlets aren't affected.