DefinitelyTyped is an open source project that maintains a repository of type declaration files for many packages, including the Google Publisher Tag (GPT) library. You can install the GPT types with npm from the @types/google-publisher-tag package.
npm install --save-dev @types/google-publisher-tag
Once installed, you have access to all of the types exposed by the
googletag object in your own code. You can also take
advantage of code completion and content assist for GPT methods
and properties in source code editors that have those features,
for example, Visual Studio Code.