The Gas Gas engines have the number stamped in the cases. The 250 4T is a Yamaha engine. I imagine gas gas would stamp them in a similar place to Yamaha (could even be stamped from Yamaha upon sale to Gas Gas).
Googled
Look on the Upper R/H side of the CrankCase,,,towards the rear.
There's a long ,narrow rectangle pad with the engine number stamped on it
It's right "behind" the kickstart lever area,,,but on topside of the cases.
Hope that helps. The VIN should be stamped on the headstem.