The FDA defines gang-printed labeling as:
“Labeling derived from a sheet of material on which more than one item of labeling is printed.”
21 CFR Part 211.122
You can think of gang-printed labeling as similar to how you would print a number of mailing address labels using a single page of labels.
The individual labels from that sheet will eventually be applied to separate envelopes for mailing.
Section 211.122 states that the:
(f) Use of gang-printed labeling for different drug products, or different strengths or net contents of the same drug product, is prohibited unless the labeling from gang-printed sheets is adequately differentiated by size, shape, or color.
Adherence to this regulation is intended to prevent accidental mislabeling that could result in the label being different than the actual drug contents.