Labelproof/rules
will failGS-01gs1

GTIN rejected for wrong length or bad check digit

AI (01) GTIN must be exactly 14 digits with a valid check digit.

Pad the GTIN to 14 digits and recompute the mod-10 check digit.

A label that trips it
^XA^BY4^FO50,50^BCN,254,Y,N,N,D^FD(01)0001234567890^FS^XZ
What the check looks for

AI (01) payload that is not exactly 14 digits, or whose mod-10 check digit is wrong.

What Labelproof says about it2 findings · GS-01 highlighted
will failLY-02^XA · offset 0 · label 1

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.

the rule on this page
will failGS-01^FD · offset 7 · label 1

AI (01) GTIN must be exactly 14 digits with a valid check digit.

Pad the GTIN to 14 digits and recompute the mod-10 check digit.

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GTIN rejected for wrong length or bad check digit — Labelproof