M_GK420T_O_A2_128_F-500x500Here is how to factory reset a Zebra GK420t Label Printer by using using the form feed button and holding it in for 4 sequences of flashing numbers.
Press and hold the Form Feed until the 4th sequence of blinks has completed. Each sequence is separated by a 1 second pause.

Factory Defaults
Resets the printer to the factory default settings (equivalent to ZPL command^JUN). Some configuration settings are not returned to their initial ‘out of box’ settings. Other settings are exclusively set, viewed and controlled by programming are also reset. The performs a standard media calibration and then a memory defragmentation routine.
Once the printer has entered the Factory Default mode, the status light will turn amber for three (3) seconds. During that time you may do two things: Do nothing and the printer will reset the factory defaults automatically as described above OR press and hold the feed button to enter a factory default reset modes for printers with a network (Ethernet, Wi-Fi or Bluetooth) printer option (equivalent ZPL command ^JUF). Releasing the button after the first flash resets the network factory options only (equivalent ZPL command ^JUN). Releasing the button after the second flash sequence (two flashes) will reset the printer defaults only. Releasing the button after the third flash sequence (three flashes) will reset both printer and network settings (equivalent ZPL commands ^JUN and ^JUF)