Look at this routine in update.cpp:
void Update::create_integrate(int narg, char **arg)
Note that to add a new integrator, similar to the Verlet or Respa
classes, you’ve really got to provide a lot of functionality to insure
that all fixes and other classes called from the intergratore are
invoked correctly and operate as expected … that’s why it really
isn’t enabled in LAMMPS with a RunStyle or IntegrateStyle setting in
style.h. If you grep the source code for “verlet” you’ll see a lot of
fixes are checking for Verlet explicitly, which you’ll need to look at.
Let me know if you’re successful adding a fully functional Gear class.