On the GasGas USA webiste, looking in the model year 2003 parts, page 102, item #4, it is called plastic counter weight, there is (2) on each side of the crank weights, and a couple of allen head bolts that secure the plastic weights to the crank.
I was kick starting my bike when it locked up, I had very small fragements of black plastic had fallen down into the crank area. I was able to wash out the bottom end and get all the bits out.
TO make this job eaiser, I removed the engine from the bike, and worked on the motor from a work bench. I was able to fish out the bolt, cut down an 90 degree allen wrench and loctie the bolt and reinstall, and secure the bolt tight. I then removed the rest of the bolts and secured them.
Or, you could split the cases, split the crank and go that route....That might be a better route if you were running the motor or riding the bike when it locked up....could be some damage to the rod??? Since I was just kicking it over, I was pretty certain very little damage was caused..if any.
Good luck....If I'm not mistaken, only the 125 and 200 are like this, the 250 / 300 don't have stuffers.