with ABS there are 'n' teeth (possibly 50 or more) per wheel revolution. I have no idea if there is a standard for this, but I suspect there may be. You can count the teeth yourself.
The gearbox speedo drive sensor comes off the final drive so will produce 'm' pulses per wheel rev. If you are dead lucky then 'n' and 'm' per wheel rev are the same. If not then hopefully 'n' is greater than 'm' by a sensible figure (like 2x, 4x, 32x etc.)
The divided signal is sent into the existing gearbox sensor pickup cable.
I have one reservation with tapping into the ABS sensor signal. The connection you make, ideally with a propper 'Y' cable, must NOT interfere with the ABS wheel sensor signal.
This sensor produces small pulses from a fairly high impedance source. This means that you pickup circuit MUST NOT load the signal. Any medium to major degradation in the ABS signal to the ABS control unit will make the ABS unit think the wheel is locked when you are breaking and thus release the braking to that wheel. Far from ideal and hopefully the remaining 3 wheels will be sufficient.
There may be a buffered and relayed ABS pulse signal to the ECU / body computer that would be safer to tap into. Grounds for more research.