This article explains exactly how Fundraise Up interacts with Google Tag Manager.
The first of the critical Tag-firing requirements is the purchase event.
When a donation completes successfully Fundraise Up passes an event to the GTM dataLayer called. -> purchase
This event can be used a trigger within GTM to fire other necessary tags.
The second important data point that most tags need is the value of the donation.
Fundraise Up passes this in the datalayer called eventModel.value
In short, with these two pieces of data passed, and that Fundraise Up is already providing DataLayer, will allow you to interact with your GTM installations in your own custom environment.
It can be used in the above manner, as opposed to the traditional sense of placing on the page.
As long as GTM is on the ‘host’ page the above method will work.