The driver for the Zebra GK420d printer is located on a disc that came with the printer in its original packaging. Insert the disc into an appropriate drive and follow the instructions in the documentation, which is also included on the disc. If the disc is no longer available, download the ZebraDesigner driver 32/64 bit. Certified for Windows. driver shown here from the Zebra GK420d Printer Support page:
You must accept Zebra's End User License Agreement (EULA) to begin the download.
Once the file is downloaded, double-click on the downloaded file to install the driver. No driver configuration is necessary.
UPDATED: August 14, 2021