Have you seen the manuals on the GasGas site? http://www.gasgasmotos.es/en/manuals.html
I am not 100% sure because I haven't even ridden mine yet but I believe the filler is above the front sprocket kind of in front of the carb and it has an allen key head.
As for the oil, I think gear oil will be too thick, the manual for the 2011 EC300 calls for SAE 10W30.
Don't use auto oil. Many here use Shell Rotolla T synth, I think it is called T6 now. I am running Amsoil 0w-40 and love it, but it cost twice as much.
The filler is just under the carb on the left side of the bike.