Connect Google sheets to different data sources

 https://dataconnector.app/

Not quite full-baked yet, but shows promise. 

API Connector lets you access data from thousands of platforms directly within Google Sheets. No coding required!

https://mixedanalytics.com/api-connector/
Interesting, instead of making google sheets an API server, it makes Google Sheets and API client. 

Sheet.Best Convert your Google SpreadSheets into APIs

https://sheet.best/#/
minimal spa

Instantly turn any Google Sheet into RESTful API.

https://sheetson.com/
Turn Google Sheets into an API

Turn any Google sheet into an API instantly, for free.

https://sheety.co/
Interesting project: turn a Google sheet into an API.

Read and write Google Sheets from PHP

http://www.phillipshipley.com/2017/05/read-and-write-google-sheets-from-php/
Read and write Google Sheets via PHP

50 Google Sheets Add-Ons to Supercharge Your Spreadsheets

https://zapier.com/blog/best-google-sheets-addons/

I admit, I've never used a Google Sheets plugin. Here's how to use them and good ones to start with: