This article is only valid for versions of Sharegate prior to 5.8. You can download the latest version to have incremental copy in Insane Mode.
When doing a pre-migration check or copying content, one of the following messages is displayed:
You selected to do an incremental copy as a merge option which is not supported by the Insane Mode. The copy will continue without using Insane Mode.
Due to technical limitation of the Azure Migration API, the incremental copy is not supported by the Insane Mode. When Sharegate detects an incremental copy using the Insane Mode, it automatically switches back to the "Copy and Replace" mode, which is the default.
The complete list of limitations for the Insane Mode can be found here
The Copy SharePoint Content tool allows you to create custom property templates when copying over content - allowing you to set certain parameters for your content migration. What you'll need to do is connect to your source list/library as well as your destination list/library in the Copy SharePoint Content tool. From here, you can select all the items in your source list and click copy.
The first pop-up you'll get after clicking on Copy SharePoint Content should be one prompting you to "Choose or Create a Property Template". From here you can click on "Create a Custom Template" and set the "Filter by time range" option to "Custom" and set the "Modified from" setting to the appropriate date range for your incremental migration. A way to do this would be to set the dates to after your original migration was done up until today. This should cause only the content within that date range to be migrated. There's also a variety of other settings that can be changed in property templates (mostly under the Metadata tab) that could be useful in the future.
The full documentation on the property templates can be found here.
You can also download the latest version to have incremental copy in Insane Mode.