I think the GS can have IBS without ABS and ABS without IBS...if that makes sense.
Basically the IBS is a current sensing circuit, so you need a ballast resistor to simulate the filament current as you say. 8 ohms will mean you draw 1.68A which will is 22.6 Watts. Your 5W resistor wouldn't last too long at that power and something will melt!
You are thinking of the brake light perhaps? The IBS senses current on the tail light circuit. If there is no (or little current) it then decreases the current flow to the brake filament to simulate a tail light. it doesn't work the other way round.
Ohm's Law is V=I/R and Power = (I^2)R
Using the above R can be found assuming 5W bulb and 13.5v battery/alt output you have:
P=IV, I=0.37A
R=V/I R=36.5 Ohms (33R or 39R)
So you will need a resistor capable of handling 5W (go for a wire wound 7W or alu clad 10W)
Parallel is the way to connect to shunt the current.
One thing though, if the unit has a separate number plate bulb (effectively another tail light), the current draw should be sufficient to fool the IBS. If it is lower than 5W try changing that bulb to a 5w version. Bear in mind the power use (i.e. effective resistance) of this bulb when calculating the shunt resistor.
