Check to see if the speed sensor is sending pulses to the gauge. Back-probe the Pink wire at the speed sensor connector (inside the headlight shell).
WIth the key On, raise and spin the front wheel. If the speed sensor is working you should see the voltage toggle between 0v to 5v.
If the sensor signal does not toggle then likely the sensor rotor is bad (easy and cheap to replace) or the sensor itself might be bad. If the sensor signal is good then the problem is at the gauge, gauge connector, or interconnect wire. Good luck.