Hi!
I found this post over on the development forum which might help in adding Google Analytics event tracking for a HubSpot form submission: https://community.hubspot.com/t5/APIs-Integrations/Trigger-Google-Analytics-Event-with-Form-Submission/m-p/223490
I also found this article which might help with the Google Tag Manager set up: Tracking HubSpot Forms with Google Tag Manager - Clarity Global
I hope this helps,
Katie Townsend
Inbound Executive
BBD Boom