I want to use the google spreadsheets API in Godot, but I don't know how to use it with gdscript. I'm new to APIs in general.
I never did (and honestly cannot imagine a situation I would), but have you checked out this question and this asset?
Start with this simplest Godot REST API tutorial: https://github.com/nyenye/godot-docs/blob/making_calls_to_rest_api/tutorials/networking/making_calls_to_a_rest_api.rst
It contains only 3 lines of code
Next could be something like Google Sheets API Guide: https://developers.google.com/sheets/api/guides/concepts