What is the main jet size and needle number?
It sounds to me like you are running out of fuel.
1. You may have a fuel restriction. I would check to see if your petcock filter or secondary filter is partially plugged with debris. Also verify there is no debris stuck between the needle and seat and the carb inlet port.
2. Low float could run out of fuel. Even though it is a new carb, I would double check the float setting is set properly for the GG bike. Start here
http://www.gasgasrider.org/forum/showthread.php?t=1427
3. Carb Angle - The carb in a GG is positioned on a complex angle that is difficult to eye properly. After you set the float height put the bike on a stand and put a bubble level on the carb top to verify it is not twisted. You may be surprised how far off the bubble level measurement is from eyeing level.