We are recommending that you contact an attorney or your local governing authority for assistance with what you must do to comply. Our understanding is that it is much more than just your interaction with ONTRAPORT.
For our part of it, you can go to the Legal Page and click on the "GDPR" link:
That section, and the section following, includes all of the information you need to show that ONTRAPORT is compliant as a data processor. Note the Privacy Shield link as well; ONTRAPORT is part of the Privacy Shield program for the EU and Switzerland, and you do not need any other document from us to make sure ONTRAPORT is compliant.