By all means go for 5 speed gear box , u will feel great diference in ur ride , ur Gto will relax after 70km ,Less vibration,otherwise cuz of 4 speed ,bike tun ma rehtee ha , and u will have better fuel economy as well .... i have done 5 speed conversions on 4 speed Gto 100 n 110. last time the 5 speed gearbox was for Rs.3500 ,and yes u can fit fuel gauge from GTO 125 ,if u cant find then ,from yamaha excellence bike , pm me ur cell no...
Regards,
Sky_ZeD