They're the different models available to Canada. The sedan is available in the GX, GS, and GT trims, while the hatch is available in GS and GT.
Pricing is as follows:
Sedan-
GX: $16,195
GS: $17,695
GT: $21,345
Hatch-
GS: $20,185
GT: $21,385
Visit Mazda.ca for more info on what is different between each.
As for GFX... that would be the appearance package, no?