It’s possible that the Gerbil board uses milliseconds instead of seconds for dwell timing.
Default GRBL uses seconds, so the pauses in your generated GCode would be G4 P0.5 (500 milliseconds is 0.5 seconds). If Gerbil is expecting milliseconds, 0.5 would be half a millisecond, and not perceptible.
Try this: Save the GCode to a file, edit the G4 P0.5 lines to be G5 P500 instead, and then run the gcode file to see if that does what it’s supposed to.