that's the compensation reservoir for the cooling system.
When your system ist full of liquid and you heat your engine up, the liquid expands.
The inner lid oft he radiator opens and the amount of liquid which is to much flows to the reservoir.
Cooling down sucks the liquid back, so the colling system always is full.
For cooling systems you should use water (some swear on distillated water). Not oil!
To prevent this from freezing (and destroing your engine) you should mix some cooling fluid in.
Here we call it 'Glysantin'. It's some kind of alcohol (Monoethylenglykol).
This one is from BASF, but you can also use something cheaper from other brands.
The cooling system of your Gilera should have about 1,1 liter of volume.
Take care that you fill up this amount.
Air locks in the cooling system may end up in a sized engine