Most developers think of Geth as an application you would run on a Desktop or Server to connect to the Ethereum Blockchain (and indeed Rivet uses Geth to do just that), but if you’re developing in Golang it’s also a library you can use to connect to an Ethereum client hosted elsewhere.
The basics of using the go-ethereum/ethclient are documented on GoDoc. You can connect to Rviet with the client library using
import "github.com/ethereum/go-ethereum/ethclient" conn, err := ethclient.Dial("https://YourApiKey.eth.rpc.rivet.cloud/")
Then use your connection as documented in GoDoc.