This ^GB is thicker than it is wide or tall, so it prints as a solid black block — and if it sits beside a barcode it destroys the quiet zone.
Reduce the border thickness, or draw the block deliberately.
^XA^FO50,50^GB20,100,30^FS^XZ
^GB whose border thickness is greater than or equal to its width or its height.
Neither ^PW nor ^LL is set. Both fall back to a value stored in the individual printer, not to a value in the spec — so the same ZPL prints differently on two printers of the same model.
Set ^PW and ^LL explicitly in every format.
This ^GB is thicker than it is wide or tall, so it prints as a solid black block — and if it sits beside a barcode it destroys the quiet zone.
Reduce the border thickness, or draw the block deliberately.