Would this be compatible on the Zebra GX420t?

The TR3101ZW has a 1" core and would be recommended for larger printers like the Zebra 110Xi4 or 140Xi4. Our TR2181 would be compatible with the Zebra GX420t. The Resin Enhanced Wax (ZW) would be recommended for the Standard or Removable White Thermal Transfer materials but if using the Weatherproof Thermal Transfer, you would want to consider the Durable Wax Resin (ZC). You can view the product page at the link below for more information. https://www.onlinelabels.com/TR2181.htm

Will this ribbon work for the Zebra ZD620? I could not find the outside diameter for this ribbon.

The outer diameter for the TR3101 is 2.75". According to the specifications of the Zebra ZD620, the maximum accepted ribbon outside diameter is 2.6".

Is this compatible with the Zebra ZD-500?

According to the specifications for your printer model, Zebra's ZD-500, the TR101ZW would not be compatible. Instead, we'd recommend the thermal ribbon below as it meets the core size and length requirements for your printer: https://www.onlinelabels.com/products/tr2181

will this work for an avery monarch 9416 xl

Our TR3101ZW is a coated side out thermal transfer ribbon confirmed to be compatible with the Zebra printer models listed below. We haven't tested Avery's 9416XL to confirm compatibility so you'll want to review your printer's specifications to ensure this ribbon can be used. Zebra® 110Xi4, Zebra® 140Xi4, Zebra® 170Xi4, Zebra® 220Xi4, Zebra® 105SL Plus, Zebra® ZM400, Zebra® ZM600, Zebra® ZT230