When copying a Nintex form that uses a control that references a specific list item id (e.g. the List Item form control), the following warning is displayed:
The list item "my list item id" associated to the list "my list" is required for this Nintex form and cannot be found at the destination. Please make sure to copy the list "my list" and all its content prior to copying this Nintex form.
When your Nintex form contains controls which use or display specific list items, Sharegate will automatically copy the structure of their containing lists, but it won't copy their content. As such, the list items that are referenced in your Nintex form won't be copied automatically by Sharegate, and your destination Nintex form may display the wrong list items.
This behavior is by design and is intended to limit the amount of data Sharegate has to process in order to speed up your migration. As an example, if your Nintex form references a list item that is contained within a list with a couple hundred thousand items, copying all the list's content would slow down considerably your form's migration. Instead, Sharegate skips the items and adds a warning to inform you that you should copy the list and the items that are referenced in your Nintex form prior to your migration.
To resolve this issue, either copy the list that is referenced in your Nintex form and all its content, or copy the list without its content (by making sure the option List Content is not checked), then copy only the list items that are referenced in your Nintex form using the Copy SharePoint Content feature. Once the referenced list items are copied, you can copy your Nintex form again.