(Disregard, problem seems to have spontaneously resolved itself. o_O )
Suddenly my printer has no nozzles and no bed?! ("Semantic warning: T0 cannot select non-existant extruder")
I'm not sure if it's your file or the GPX plugin, but it looks like the GPX translator is sending some stuff your firmware can't understand. It may just be in a bad state...have you tried restarting OctoPrint? What firmware are you running?