When copying a Nintex Form, Sharegate displays the following warning:
This form (...) references Form Variables that are not supported at the destination: Variable1, Variable2. The form will be copied but it will not behave as expected until it is updated.
Nintex Forms variables are currently only supported when using SharePoint 2013 and SharePoint 2016 on-premises. Sharegate will give this warning when trying to migrate to any other type of environment.
At the destination, edit the Nintex Form and change all references to the specified form variables. The warning message will contain the control and/or parameter which contains the variable and requires a manual input.