I ran a quick experiment where I measured the value of relative gamma band power. You'll find the charts I generated below for each electrode. Baseline was with a steady LED no flashing while gamma was with the LED flashing at 40 hertz.
The increase of relative gamma in FP1 and FP2 with the flashing LED is so clear-cut I think it might be signal pollution from the LED's. I'll have to run another experiment to make sure. I also think the recording for TP9 might be a bit off due to the sensor not being properly placed.
Anyway that's a quick little initial experiment. The results surprised me a bit I have to say. I would like to give this experiment another go to make sure the effect is genuine.