When attempting to copy an item, Sharegate displays one of the following error messages:
An unexpected error occurred: Save Conflict.
The file 'My File' is modified by 'Someone' on 'Current date/time'
This error normally happens because there is a workflow or an event listener set to run automatically when an item is created or updated. Since Sharegate is trying to update the item at the same time as the workflow or event listener is updating it, it causes save conflicts and it prevents Sharegate from updating the item.
Go in SharePoint in the destination list or library settings and temporarily disable the workflows that are set to run automatically when an item is created or updated. You can then copy the items with Sharegate and, once the copy is over, go back in SharePoint to set the workflow settings to their original values.
If you have event listeners that modify new and updated items, these will also need to be disabled during the copy.