When attempting to copy multiple site collections, Sharegate displays the following error message:
The site collection could not be copied because more then one site collection would result in the same destination address: 'http://mySharepoint/sites/TeamSchedule/'.
The error occurs because at least 2 site collections from the source would have the exact same address at the destination and would be merged in the same site collection. In order to avoid this, both site collections won't be copied and will give this error. For example, let's say you have a site with the following address 'http://mySharepoint/sites/TeamReporting/' and another site with 'http://mySharepoint/Teams/TeamReporting/'. When you migrate, you choose to the default url '/sites/' to create all site collections under this path. When you migrate the two TeamReporting sites, they will end up with the same address which is 'http://myNewSharePoint/sites/TeamReporting/'. Sharegate will prompt an error since it cannot know if you really want those two sites to be merged.
To avoid the path collision, migrate each site collection individually and specify a different address or managed path. If you really desire to merge your multiple site collections into one, you simply need to migrate each one toward the same destination address and managed path, in two separate operations.