Where did you order your board from? Is it an official version of the alexmos gimbal controler? Usually the error is related to a missing bootloader:
avrdude:stk500_getsync():not in sync: resp=0x00"
So by the sounds of it either you are not using an official board (so you cant update the firmware) or your bootloader as become corrupt for some reason.
Otherwise the other issue could be you are not selecting the correct COM port, or you dont have the correct drivers installed.