It is strongly recommended to remove the Read-only lock on your source site collection prior to the use of Sharegate in order to ensure that all the requests are processed as intended. Some SharePoint services do not respond correctly when the site collection is in read-only mode (even if they are only used to read information).
Not removing the read-only lock will mostly affect the structural parts of your environment, like Web Parts and Site settings, but it is unpredictable and 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 the above 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.
Removing the read-only lock on your source site collection prior to the use of Sharegate ensures that there are no related issues when performing your migration.