Gem lets you buy and sell NFTs across multiple marketplaces and makes it easier to sweep the floor of an NFT collection (that is buy a range of the lower end of the collection). Following the acquisition, Gem will continue to operate as a standalone brand.
Gem said the acquisition offer was "unexpected" but that it would accelerate its next stage of growth. It added that Gem would be gaining access to OpenSea's infrastructure, resources and distribution channels.
https://www.theblockcrypto.com/post/143297/opensea-acquires-nft-aggregator-gem