Probably should be in one of the other forums - it may get moved!
Anyway, you need something like an
Acumen DG8 - this compares the tach signal with the wheel speed signal & calculates then displays the appropriate gear.
I'm not sure if the 250 Ninja has electronic speed signal? Think it might be mechanical - cable drive? No matter even if it is - you can add an a
uxiliary speed sender to provide the input for that.
Not difficult to install or wire up.
You can get in the US from somewhere like
RiderStation
Edit - I see the 08 has a new speedo, possibly electronic drive which would make it easier - you just need the DG8 in that case - universal model most likely.