google-login-with-react | A minimal example to integrate Sign in with Google | Authentication library

It looks to me like you're loading the google apis as a script tag, which we'd expect to set window.gapi to the google api. However, you're then running eslint or some other checker, and that has no idea that window.gapi is supposed to exist. It's failing because it sees you using an undeclared variable.

A cheap fix is to tell Eslint that you know what you are doing;