A gateway drug ASIN is the first-purchase product identified via the DSI framework as generating the highest average downstream (repeat-purchase) value — i.e., the product most likely to turn a first-time buyer into a high-LTV repeat customer.
Once identified, the targeting strategy is to concentrate resources on that ASIN rather than spreading spend evenly across the catalog:
The bet is that acquisition cost on this ASIN is justified by downstream value elsewhere in the brand's catalog, not by the unit economics of the ASIN itself — a variant of thinking used in Brand-Family Line Extension for LTV and Customer Lifetime Value (LTV) as the Primary Amazon Margin Lever.