Proper wiring for GFCI given switched circuit
Just wire the GFCI between the wiring and the disposal so that it's on the switch (often, you would daisy chain the switch off of the GFCI and have the switched device hardwired, but that doesn't solve your problem).  So, in your diagram, the load side is where the current is coming in, the line side goes to the switch, and the disposal is obvious.  What you'll have is:

              Switch (I'm guessing this is "line?" in your diagram)
               | | |
Line           W G B           GFCI   Plug  Disposal
Black ---------+ | +--B------- Black  =----B
Green -----------+----G------- Green  =----G
White ----------------W------- White  =----W


In the switch, the input is the white, even though it's hot in the wiring, and the output of the switch is black, since that's what becomes hot to the next device.  When possible, I like to have my switch connected to a black and red wire, black always being hot, and red being switched.  But that's usually not convenient, so this is the best an electrician can do.