Gliffy Diagrams are stored in JIRA as an attachment; so you can try to narrow down your search by using the following JQL.
attachments is not EMPTY
This will however return all results that have an attachment and not gliffy only.
Alternatively you could ask users to add a gliffy label to any issue they add a diagram to to increase tracking then search for the label with:
labels = "gliffy"
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