When I've had to hold off of a GPS or FMS point, it has usually been given as "Hold present position" or "Upon reaching such and such point, hold". Sometimes they'll add in a leg length and a turn direction, but if not then I just use timed holding and standard right turns. In either of these cases my "inbound" course of my holding pattern is just whatever course I happened to be on when they gave me the hold (for hold present position), or my course TO the point they told me to hold off of.
So if I'm flying from point UMPTY to point FRUMPTY, and I get "hold present position", you use the course from UMPTY to FRUMPTY as your inbound. If instead they tell you to proceed direct to FRUMPTY and hold, then you just use your course from present position to FRUMPTY as your inbound (assuming of course that there are no PUBLISHED patterns at FRUMPTY).