In retail trade, a distinction is made between a product as a single item and a product as a "sellable unit". The product ID for packs, sets and bundles should represent the correct single sellable unit.
Add the product ID that represents the quantity of items contained in the package that you are selling.
Example: A single pair of gloves will have one product ID, but a pack of three pairs of the same gloves will have a different product ID.
Bundles are composed of multiple different items that would each require a separate GTIN if sold separately.
When shipping items to retailers, manufacturers use a GTIN-14 to designate the smallest sellable unit and the number of those units contained in a single case or box. The GTIN-14 identifies a "case pack" that can be sold as a unit. When selling an entire case, use the GTIN-14 as the product ID rather than the UPC for any single item contained in the case.