haha, I’m an Apps Script junkie 
I was a GSuite user long before I became a Glide user, so I have an Apps Script function for just about everything you can imagine. Every time I start a new project, I drop a few hundred lines of utility functions in before I even get started.
But yes, as Jeff mentioned, Apps Script is a strict no-no when it comes to Glide templates. And the same applies with CSS, although there has been a recent hint that this at least might change sometime soon.