Sharegate does not allow you to manually edit the address in the import to SharePoint File explorer so you can't access directly a Network drive by its address. I.e. \\server\share. To access a file share in the explorer, you need to map the network drive on you computer prior to use it. To do so, open the windows explorer and select "My Computer" in the left pane. Then click the "Map network drive" option in the ribbon.
In the Map Network Drive window. Select a letter and enter the network drive address
Once the folder is mapped. You should be able to see it in Sharegate. You might need to click on the refresh button if the page was already opened.