avrdude: stk500_getsync(): not in sync: resp=0x00
this thing works for me. Remove your GSM Shield first before you upload the sketch or unplug and plug your usb port connecting in your arduino check also the hardware usb ports in device manager be sure it's updated...
if(gsmAccess.begin(PINNUMBER)==GSM_READY)
try to use this library because the one you use eats a lot of memory.