Most GFI outlets have an extender capability built in so you can control another non-gfi outlet from the breaker in the GFI outlet. Look at the instructions from the GFI receptacle and wire accordingly if you want the garage outlet to be GFI too. If you don't, just connect the garage outlet to the GFI receptacle's input power terminals. There should be a diagram in the instructions. Typically they are also labeled on the back or side of the GFI receptacle.
This is assuming that the Garage wires go through the same box as the gfi receptacle. It is pretty simple really.
