How can I fit a face plate to two devices in separate single-gang boxes?
The gap you are seeing is well within the normal limits of adjustment for standard yokes (the metal thing the screws go through).  

You just tightened down the yoke screws and let them land where they may.  You'll need to back them off, shift the outlet and switch around as needed, and repeatedly try the plate until it lines up.  

If the yokes don't give you enough horizontal slack in their slots, you may need to change to different devices that do.  

If all else fails, roll back to the original receptacle and switch, figure out from where this location is supplied, and install the GFCI there with this part of the circuit on the LOAD terminals. If there's already a GFCI there, then this one is redundant.