Suddenly my printer has no nozzles and no bed?! ("Semantic warning: T0 cannot select non-existant extruder")

(Disregard, problem seems to have spontaneously resolved itself. o_O )

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?