You can use this public endpoint which is a read-only list of all Grant Names & Ethereum Addresses: https://gitcoin.co/grants/grants.json

Also check out these docs on the smart contract architecture of Gitcoin Grants: https://github.com/gitcoinco/web/blob/master/docs/GRANTS.md

and the Gitcoin Grants API:

https://github.com/gitcoinco/web/blob/master/docs/API.md