Hi, developer behind Check Sheet here.

Check Sheet is a free add-on for Google Sheets that adds the ability to define rules that will trigger an email notification when your spreadsheet changes. For example if a certain cell exceeds a value or if all values in a column match a string.

Google Sheets does have very basic notification functionality built in but it can only trigger notifications on any change, you can't narrow it down further than that.

This is the MVP I've built in a couple of months, any feedback is much appreciated.