The exact difference in clock rates is not constant, because the orbits are not perfectly circular and the Earth is not a perfect sphere. So both the altitude and speed of the satellite, and the Earth's gravitational potential, are varying with time, and that means the clock adjustments will vary with time as well.

For the GPS satellites, their time signals are constantly compared with ground clocks, and adjustment signals are sent up to the satellites as needed to keep their clock corrections in sync with ground clocks.

I'm not sure what, if any, adjustments are made to clocks on the ISS, or how they're done.