Two items I can think of that are not specific to GasGas:
Improperly adjusted lever/improperly bled system. Not having full travel from the lever means that the clutch isn't fully engaged, so parts are fighting each other for space. Check lever free play, there should be very slight movement between loose and engaged. The tip of the lever should move a couple of millimeters, not a couple of centimeters.
Or it could be that your outer basket fingers are notched. When the fingers look like this-
The fiber plates hang up and fail to properly engage/disengage. Pull the cover and look at the fingers if the clutch lever adjustment doesn't help the problem.
That being said, many bikes have an inherent "clunk" when engaging first gear, but it can be excessive or a sign of a worse problem.