Adding the tracking code via Google Tag Manager follows the same process as adding any kind of HTML code to your website:

Add a “Custom HTML Tag” product
Paste the tracking code into the “Custom HTML” text box in the “Configure Tag” step
Choose “All Pages” in the “Fire on” step to have the code run on all pages

In case you want the code to run on specific pages (e.g. landing pages for your protected campaigns) select the “Some Pages” option and define the rule in step 3.