(Ideal) Amplifier networks/nullors - determining the gain
Hmm, never heard of a Nullor before.  Go figure.  Sometimes it is easier to think of these things in terms of control systems.  The amplifier tries to set the two input voltages equal to each other.  One input voltage is the current command.  The other one is the voltage across R1.  So what it tries to do is set the voltage across R1 to the input voltage.  The voltage on R1 comes from the current flowing through R1.  The amplifier will increase the supply to the load until the voltage across R1 is equal to the input.  In this case, Iload = Vin / R1.  This circuit is called a transconductace amplifier because the gain is actually Iload / Vin = 1 / R1, which has the units of conductance.