At this moment, there is isn't a way to mass import Gliffy diagrams to diagram in Confluence Cloud. Each diagram would need to be manually imported by saving the Gliffy diagram attachment (see the page attachments) to your file system, inserting a new diagram to the page (remove the Gliffy macro) and once in the editor, drag and drop the saved Gliffy file into the canvas to import.

You can find instances of your Gliffy macros at