There is a very tiny difference of power b/w CG and GS and given the fact that it has best power to weight ration, one wrong gear shift and CG will eat a GS alive and will not BURP!!!
The fact that you got stucked at 6.5k was (I doubt) a bad shift .. If I am riding with this heavy laden load and plan to redline a bike I will not change any gear below 9k RPM.
Still, GS150 would have performed well, but there can be a reason why did it not performed :
1 - Is it in running period ? if yes then stop racing...
2 - If it to lean or Too Rich ? get it checked ...
3 - Tyres are inflate ?
4 - Poor engine health... (roughly aik horse power ka farak hai GS thori see bhi down ho health men tu CG usko pharka deti hai...)