As with many of the cool, useful tools on the internet these days, it's something that Zach Leatherman built. Thanks Zach!
The thing - though - is that glyphhanger requires Puppeteer.
I use Windows for all my day-to-day programming outside work, but I use Linux - via WSL2 - to actually make it a usable environment.
Puppeteer doesn't work out of the box in a "headless" Linux environment.
Thankfully, I found someone who had the same problem. They went through a lot more steps, so I thought I'd boil it down to just 2:
sudo apt install -y chromium-browser
sudo apt install -y libatk-adaptor