The GK420d is a direct-thermal desktop model commonly retained in shipping rooms long after the GK series was discontinued. Its native ZPL II path makes it a practical target for raw command printing.
Specifications and steps reviewed 17 October 2026.
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Successful jobs for this exact reported model exist in LabelInn production telemetry. The connection, firmware, LabelInn version, and date were not captured unless stated below.
Production telemetry contains successful print jobs reported as Zebra GK420d; connection type and firmware were not captured.
legacy 4-inch shipping and inventory stations. The GK420d is a direct-thermal desktop model commonly retained in shipping rooms long after the GK series was discontinued. Its native ZPL II path makes it a practical target for raw command printing.
The GK420d is a direct-thermal 4 in desktop printer. Zebra's data sheet lists ZPL II, EPL2 page mode and EPL line mode, 203 dpi, a 4.09 in (104 mm) print width, programmable speeds of 2, 3, 4 and 5 in/s, and RS-232 serial plus USB as the standard interfaces. An internal 10/100 Ethernet print server was an option that replaced the serial port. The data sheet dates from 2009, so most units you meet today are long-serving ones.
Hands-on record: LabelInn engineering keeps a GK420d on a Windows PC over USB, and it appears in LabelInn as "ZTC GK420d". On 2026-10-15 we unplugged it while idle and while a job was running. The app marked it offline within 0.8 s, recognised it again 2.1 s after the cable went back in, and a job that had been waiting for it printed by itself about 2 s after reconnect. Those were connection and queue tests on one unit; they are not a certification of every GK420d, and we have not tested it on a Mac.
| Variant | Direct thermal. The thermal-transfer sibling is the GK420t, a separate model. |
|---|---|
| Resolution | 203 dpi (8 dots/mm), dot pitch 0.0049 in (0.125 mm) |
| Maximum print width | 4.09 in (104 mm) |
| Media width | 0.75 to 4.25 in (19 to 108 mm) |
| Label length | Tear-off mode: 0.38 in (9.7 mm) minimum on the GK420d, 0.5 in (12.7 mm) with the dispenser. Maximum 39 in (990 mm). |
| Roll | Fully enclosed media compartment for rolls up to 5.0 in (127 mm); core inner diameter 0.5, 1.0 or 1.5 in |
| Print speed | Programmable 2, 3, 4 and 5 in/s (51, 76, 102, 127 mm/s) |
| Media sensing | Transmissive and reflective sensors, head-up sensor, auto-calibration at start-up |
| Command languages | ZPL II, EPL2 page mode, EPL line mode (GK420d) |
| Interfaces | RS-232 serial and USB (V1.1) standard. Optional internal ZebraNet 10/100 Ethernet, which replaces the serial port. No Wi-Fi or Bluetooth is listed. |
| Bar code module | 5 to 50 mil at 203 dpi |
| Memory | 8 MB SDRAM (3 MB available), 4 MB flash (1.5 MB available) |
Sources: Zebra GK420d and GK420t Printer Specifications, #14268L, 30 Sept 2009 (PDF, copy hosted by a reseller); Zebra GK420d user guide: Feed button modes; Zebra Feed Button Modes for G-series and Plus-series printers (PDF); Zebra manual calibration procedure
A GK420d has no ribbon, so its ribbon sensor always reports empty. LabelInn knows this: ribbon-out alone does not stop a job, it only counts when the printer is also in an error, paused or offline state. Set Print Method to Direct Thermal so LabelInn writes ^MTD rather than the ^MTT of its transfer preset.
LabelInn's Darkness field is Zebra's ^MD value, 0 to 30, and a job's ^MD overrides the value stored in the printer, as Zebra says of drivers and applications in general. Zebra's own darkness test (six flashes) prints a pattern and tells you to stop as soon as it is legible: past that point the bar widths distort and scanning gets worse. Begin at the preset 15.
The GK420d tops out at 5 in/s. LabelInn's built-in thermal presets write ^PR6, so set Print Speed to 5 or lower.
Feed Mode becomes ^MNY for gap or web, ^MNM for black mark and ^MNN for continuous. A gap setting on black-mark stock causes one-label skips. GK420d not printing or skipping labels walks through the checks.
Only units fitted with the ZebraNet 10/100 server have a network port, and that server replaces the serial port. If yours does, raw printing uses TCP port 9100. Every unit has USB.
In the desktop app an unreachable printer is a delivery state, not a failed job: a job started from the print dialog while the GK420d is unplugged or switched off waits and prints when the printer returns. Jobs that arrive from the cloud or from a workflow while the USB printer is unplugged are not covered yet and can fail after a few attempts. Small ZPL jobs reach the printer's buffer in about a second, so a cable pull rarely cuts a job in half.
A 4 x 6 in (101.6 x 152.4 mm) shipping layout drawn in printer dots: 812 x 1218 pixels at 203 dpi, with a Code 128 barcode at a 3-dot module (0.37 mm). The label is 20 dots narrower than the 832-dot head.
Media preamble LabelInn writes before the label for this size (direct thermal, gap, darkness 15, speed 5)
^PW812^LL1218^MD15^PR5^MTD^MMT^MNY^LS0Use either the operating-system print path or direct protocol output when this record marks direct printing as supported.
Direct protocol output avoids a model-specific print driver when the selected connection is available to macOS.
Android support is conditional on a supported USB, Bluetooth, or network path; the production observation did not record which path was used.
Successful jobs for this exact reported model exist in LabelInn production telemetry. The connection, firmware, LabelInn version, and date were not captured unless stated below. Production-observed does not mean physically tested by the LabelInn team. Confirm the exact suffix, installed options, command mode, DPI, and media.
Not for LabelInn's direct command path when a supported interface is available. A vendor driver may still be useful for configuration and diagnostics.
Media from 0.75 to 4.25 in (19 to 108 mm) wide, with a print width of up to 4.09 in (104 mm) and labels up to 39 in (990 mm) long. The common 4 x 6 in shipping label is 812 x 1218 dots at 203 dpi.
Zebra's data sheet lists USB and RS-232 serial as standard. An internal ZebraNet 10/100 Ethernet server was an option that replaced the serial port. No Wi-Fi or Bluetooth is listed for this model.
With the top cover closed, hold the Feed button until the green status light has flashed twice and release it for a standard media calibration. For pre-printed media or a printer that will not auto-calibrate, hold Feed through seven flashes for a manual calibration. These steps are from Zebra's Feed Button Modes document.
203 dpi (8 dots/mm). A design drawn for 300 dpi prints at the wrong physical size on it, so set 203 dpi before you design.
No. LabelInn sends ZPL II directly over USB. Zebra's driver and utilities are still useful for configuration and diagnostics, and if the driver is installed Windows lists the printer under its driver queue name.
LabelInn's production telemetry contains successful jobs for the GK420d, and on 2026-10-15 the engineering team ran connection tests on one USB unit on Windows: unplug and replug detection in under 3 s and automatic printing of a job that was waiting in the print dialog queue. We have not tested it on a Mac.
Start with a test label, then move to Excel/CSV batches, serialization, and automated workflows.
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