Some time ago, Google has changed their Sheets "app" to expand its capabilities to allow even larger Sheets to be created and used. Unfortunately, when they did this they also introduced the Add-Ons menu across Sheets, Docs, and Slides. While this has some cool features, it also "killed" the use of many of the powerful scripts that people had developed and shared (I love the Google sharing community!).
However, I have some good news for you. It is still possible to use the script gallery to leverage the power of these scripts and others if you are using an Old Style Google Sheet. "But how do I get an old school sheet to use? Google only allows me to create Sheets that use the New Style." Well, I am glad you asked.
If you have need of an Old School Google Sheet so that you can use the "old" script gallery, click here. By clicking the link, a new window or tab will appear and allow you to make a copy of an Old School Google Sheet so that you can use the "old" script gallery to continue enjoying the powerful scripts available.
Tip: Once you have made a copy using the link above, you may want to name and store this in a way that you are reminded to make a new copy of the sheet before using it so that you always have access to an old school sheet.