I have a GFCI circuit in a garage (drywalled and well kept), at the far end of this circuit someone has run a switch leg from the second to last plug in the circuit to power an interior light in a small room attached to the garage. When the breaker is switched on the GFCI trips, is there a way to make a switch work and not trip the GFCI when the circuit is energized. I am trying to avoid running a lighting circuit back to the panel.

The switxhed box should be wired BEFORE the GFCI outlet. It is kind of defeative to do this though. The GFCI circuit is there for a reason. I personally would not "double up" on a line that was being protected. It defeats the purpose.

If you switch a light from the load side, then the neutral for the light MUST also be on the load side.
Then the light will also be protected.
Or both from the line side.

If your GFCI is tripping when the light switch is turned on, you have an "issue" with the wiring.
If circuit is wired properly even under the scenario you describe when the light switch is turned on the breaker should not trip.
Before you install another circuit figure out the problem.
Most likely it is a short somewhere in wiring (a nail puncturing a wire, the light improperly wired, a neutral wire hitting the ground etc, etc)
Be careful though, since the GFCI is tripping there IS a problem. Finding the problem before putting a new circuit in should save a lot of time and expense.
Best, Steve

I tend to think Strat nailed this one. I've seen it before. The neutral and hot for the light must either both be on the load side for GFCI protection or both on the line side. If one wire is on the line side and the other is on the load the GFCI will trip and is doing exactly what it was designed to do.