I'm familiar with the early GasGas 4-strokes.
This problem came up several times, especially in the 2005 FSE450.
Every time it happened and I was contacted, it was with nearly new or new bikes.
Then, the cause of the problem was blowby from the rings not being seated properly. The owner had started off using a top grade synthetic oil and the extra lubrication properties of the oil had not allowed the rings to seat properly. The resulting blowby would not only blow oil smoke out the breather, but was actually depositing oil in the airbox.
With the new bikes, we were able to fix the problem by going to a non-synthetic (but good grade) oil long enough to get the rings to seat properly. It would stop the blowby, and the "oil in the airbox" problem. Then, the owner could go back to their preferred lubricant.
Taking this line of thinking further in a logical manner, and taking in consideration the probable amount of use your bike has had since it was new, I think that you are most likely ready for a top end job, or at least a new set of piston rings to stop the blowby.
Have you done a compression test to check how well the rings are sealing?
This just may be your problem.
Good Luck with your bike. The 2005 was my favorite year for the early GG 2-strokes. The slipper clutch was nice, and the harder you pushed the bike in the tight trail, the better it responded. Except for the weight, if felt quite a bit like the GG 2-strokes.
Good Riding and Good Wrenching!
Jim
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