This walkthrough will let you build a tracker for email opens using Gmail, Google Sheet, and Google Apps Script.
Interesting solution that popped up in the Google Apps Script community for logging email opens using Google Apps Script. The post includes a detailed walkthrough for setting this up as well as some caveats about reliability.
Source: Tracking Email Opens with Gmail, Sheets, and Apps Script
Member of Google Developers Experts Program for Google Workspace (Google Apps Script) and interested in supporting Google Workspace Devs.