You can install @types/gtag.js. See https://github.com/DefinitelyTyped/DefinitelyTyped/tree/4d4da2106dcd54af4a0ddeb5e0360f322fd8a5bf/types/gtag.js
yarn add -D @types/gtag.js
or
npm install --save-dev @types/gtag.js
Depending on your tsconfig, you might need to add it to the types option.