Symptoms
When migrating a content type, ShareGate Migrate displays the following warning:
The content type name '$Resources:My_ContentType' was replaced by '$Resources_My_ContentType' because it contains some illegal characters.
Error code
ST-158-018
Details
This warning occurs when a content type has an invalid name. Most of the time, the reason for this is that the content type name was localized at the source, so its name was set to an entry in a resource file. If the resource file cannot be found, SharePoint displays the name as "$Resources:Resource_Name".
Solution
If the name that was assigned by ShareGate Migrate is correct, then you can ignore this warning. If not, there are 2 possible solutions:
Rename the destination content type after the copy.
Make sure the resource file is available on the source server, so that the content type's name is displayed correctly. This will allow ShareGate Migrate to migrate the content type with the displayed name.