How does the wheel speed sensor in a Boeing 777-300ER work?
From this Aviation Studies project report of group 2A2I at the University of applied Sciences in Amsterdam, for a Boeing 777-200:


  Every wheel is equipped with an anti-skid transducer (appendix VII),
  which is installed in the axle. The transducer consists of a rotor and
  stator. According to the speed of rotation, an electrical signal is
  send to the BSCU.
  
  This computer compares the rotation speed of all the
  wheels and detects when a wheel is rotating slower than the other
  wheels on the MLG. The BSCU will send a signal to reduce braking power
  when the rotation speed of one wheel reduces more in comparison to
  other wheels.


The principle is the wheel axle drives a permanent magnet within a 3-phase winding. The frequency of the signal generated is proportional to the wheel angular speed.

From B777 General Familiarization Manual:


Antiskid and Autobrake Diagram, source

Note: I initially believed the tacho-generator was a Socitec ST80, the same than on A320 (part number is C20105000). However when I wanted to provide a supporting document, I couldn't. So please take this as an assumption only until someone provides the part number from a Boeing document (I don't think there would be a big difference between B777 and A320 tacho-generator anyway).



Tacho-generator, part C20105000, source

More information: Bodet Aero. According to Bodet Aero, their tachometer equips A300, A310, A319, A320, A321, A330 and A340.

More about tachometers: TheAirlinePilots (section "Three Phase AC Electrical Tachometer").

This is a typical installation of the transducer (not a particular aircraft):


Source