"Mysterious" valve
 

Another potentially stupid question, but what is this valve for? More inportantly - can I remove it?

It is an EGR valve for emissions control. It comes with the Euro emissions kit , one end goes to the exhaust pipe and one end into the air box. You don't need it, take it off. You should open up your power valve cover on the the same side (left) and check to see if the power valve is the right one. If it is black anodized then the dealer did not put the right valve (silver) in. This would give you poor performance. The black valve is solid with no cutaway.

In know my eyesight keeps going downhill...

That hose going to the mystery valve appears to be connected to the lower radiator hose. I don't see a hose/tube connected to the exhaust pipe, which would be expected for an EGR valve.

Can you confirm where the ends of the hoses/tubes go from the mystery valve?

optical illusion ... take another look. The egr valve tube just blends in with the rad tube. does notactually connect

It's actually a fresh air valve.
It allows fresh air to be blended in the catalytic convertor to complete the exhaust gas burning process.

Indeed, the "output" pipe is not connected to anything - neither the exhaust, nor the radiator pipe. It's just left on the loose. The input was somewhere under the seat, i.e. inside the airbox. Thank you very much for the assurance that I can reduce the wieght of the bike by removing this "system".
I'll also check the colour of the power valve.

One question more: are there any limiters or something in the carburator I should remove also? The bike came with additional jets and needle, but I still haven't opened the carburator to see what's inside.

P.S.
This is the first (and so far only) GasGas bike of the "dealer" I bought it from, so we're on the same spot in the learning curve.

The original pipe has a spigot welded on for the fresh air inlet.

Didn't they at one point come with, maybe not fitted, an airbox restrictor?

There is also a throttle limiter underneath the carb top, if you see a black tube under the top remove it. Yes there is an airbox restrictor plate in the air filter, it is a biege color palte with three small holes in it, remove it also.

There is a weird-looking plastic piece with a hole in it, which I've concluded was the airbox restrictor. But it was outside the bike.
Actually, I think the bike arrived half-homologized. It had all the lights on it, but in the crate there was a note that "due to logistic problems, GasGas aftermarket department will send me later the homologation exhaust pipe, the sparkplug wrench" and some other stuff. More than a month later I'm still waiting for those, by the way...

I'll check for the throttle limiter anyway. Thanks. I doubt that there is one, but you never know.