The new Atlassian App Migration Platform (currently under development) will enable all Atlassian Marketplace Partners apps to build automated migration paths for their apps. Gliffy Confluence Server/DC migration to Gliffy Confluence Cloud mostly works as-is with normal diagrams, but there are some issues with linked diagrams and we are investigating a potential fix for those.
When a customer migrates projects from Confluence Server / Data Center to Confluence Cloud using the Confluence Cloud Migration Assistant, the app migration platform will coordinate with Gliffy to automatically move Gliffy for Confluence Server/DC data associated with the migrated projects.
While there may be a migration path, the process has some manual steps, which takes time and may cause mistakes. For specifics, please see https://help.gliffy.com/confluence/Content/GliffyConfluence/Migrating-server-cloud-Confluence.htm