To install Survicate on your website using Google Tag Manager, you will need a Workspace key:
Copy it and go to your Tag Manager.
Click on the Tags > New > Tag Configuration > Survicate Widget and paste the key into Instalation Code field in GTM
Click on Triggering and select "All Pages" > Save and Submit!
The rest of targeting is made via Survicate panel in "Settings" tab of a survey.
More on that here.