Hi trampoline,
You basically just have to edit the correspoding thml file in GL. For example, if you want an add after each article, you add something like this at the end of
layouts/mylayout/article/article.thtml:
<div class="banner">
<script type="text/javascript" language="JavaScript">document.write(getBanner());</script>
</div>
In our case, I wrote a JS function that randomly picks a banner from an array. Here’s the source:
https://www.afp548.com/javascript/banners.js