[MattR]

I don't think you listed the VIN correctly. I think the GasGas bikes start with V and not U. Also, the EC259 should probably be EC250. In any case, I'm guessing this bike is an 2001 EC250 (most likely a XC250). The traits that Fury mentions like airbox door on left side also existed on the 2001. The bikes can with Ohlins rear shock and WP inverted forks (like on KTMs). You can tell if it has the heavy flywheel (EC) or lighter flywheel (XC) by looking at the exit location of the wire on the left side cover. If the wire exits in the 2 o'clock position then it is an EC250. If the wire exits in the 10 o'clock position, then it is an XC250. Can you get some pics of the bike? That would be easiest.

EDIT: here are 2 pics of my old bike with these items identified.