SharePoint Online (SharePoint for Office 365) supports the special characters # and %.
To activate this feature:
Install the SharePoint Online Management Shell
Connect to SharePoint Online using Powershell
Launch the following command:
Set-SPOTenant -SpecialCharactersStateInFileFolderNames Allowed
Note: This procedure takes several hours to activate in the environment.
To enabled this feature:
Open the Sharegate Migration app, and access the Illegal Characters section in Options.
The Allow Office 365 special characters option must be checked.
Note: You must run a migration to test if the feature is properly activated. There is no other way to know at this time.
If you try to preserve these characters in an environment that does not allow it, you will receive an error when migrating.