When trying to migrate a survey that does not have any questions, Sharegate displays the following error message:
The survey contains no questions.
This happens when a survey was created with any number of questions in it and users responded to that survey. The survey was then edited and questions were removed from it. When trying to migrate it to SharePoint, since there are answers, SharePoint tries to map the answers to questions that no longer exist in the new environment. Since the original questions cannot be retrieved, the initial state of the survey cannot be recreated at the destination.
There is no way to recover the deleted questions on a survey, however it is possible to create a survey at the destination and merge the answers from the source. Unfortunately, the answers will be lost, but you will be able to know who answered the survey.