When editing your site in Siimple, click the “Settings” tap at the top of your editor, scroll to the Website Analytics field and select Google Tag Manager from the dropdown.

You’ll see an additional field appear asking for your Container ID — you can find this in your GTM account. For more info on setting up a GTM account: https://support.google.com/tagmanager/answer/14842164

After you’re done, save your settings and publish your site, then GTM will be automatically installed on your landing page.