With Creator Studio, you can easily convert your Google Slides presentation into animated GIFs and video slideshows. The add-on can also extract speaker notes from your slides and export them as a text file in Google Drive. Internally, the app uses Google Apps Script to export Speaker Notes from your Google presentation and writes them to a text file inside Google Drive.
In this post Amit Agarwal share some very clean code for iterating across each slide in Google Slides and export the speaker notes into a single file. Click through to the post for the source code.