I'm curious about this too. From what I've seen I think it is unlikey. The stock headlight operates on AC. The rest of the bike is DC. The 8" race light (HID) lists a power consumption of 35W and Trailtech states their HID lights require DC power (and correct polarity).
Trailtech goes on to say that a KTM 2K3 produces 55W of AC power. If the GasGas is similar and runs a stock 35W headlight, that's only 20W left for DC work. This looks too low to handle the race light and still charge the battery.
Putting a rectifier on the 35W AC headlight wiring might achieve what you want. When I did some experimenting on a KTM RFS engine (with a full wave bridge rectifier), I got 17VDC.
I didn't pursue it any further as it looked like I needed to use a regulator/rectifier to reduce the excess voltage and I didn't have one on hand.
