The Read-only lock should be removed on your source Site Collection before running a migration action with Sharegate.
Some SharePoint services do not respond correctly when the Site Collection is on read-only mode (even if they are only used to read information), and the requests will not be processed as intended.
You must remove the Read-only lock on your Site Collection before running your migration. You can do so by following the instructions outlined in this article.
Not removing the read-only lock will mostly affect the structural parts of your environment, like Web Parts and Site settings, but the behavior is unpredictable and is may affect more overall.
Sharegate accesses SharePoint through multiple APIs and has to send many requests to the servers to obtain and resolve your data. Given the complexity of this operation, the read-only lock will often cause one of these calls to be unresolved, and Sharegate is unable to push the data properly. This results in errors, and some items may not copy properly.