If you haven’t already heard about Hotjar, you should really take a look at it. The tool doesn’t only allow you to create heatmaps, but allows you to view things as scroll depth, create funnels and ask your visitor questions. And it also allows you to view recordings of visits to your pages, which is a feature I really love.
And since Hotjar requires you to add its script on the page, why not do it via Google Tag Manager? That is really the purpose of using a tag management system, right?
Once you have your account setup, you will get a tracking code that looks like the one below that you need to insert on all pages on your domain.