Sorry to see that your question has been sitting out here in the Community without a response for so long. To answer your question, I don't think Google Forms are supported in Google Cloud Assignments using the Google LTI external app. This is confirmed in the first paragraph of the blue box in the following Guide: How do I create a cloud assignment with a Google D... - Canvas Community. Only Google Documents, Spreadsheets, and Slides are supported.
I know this isn't the answer you were probably looking for, but I hope that it has helped to answer your question...thanks! Take care...be well.