homebrew-cask/doc/cask_language_reference/stanzas/gpg.md

gpg

This is a stub for upcoming functionality, and is not fully documented.

The gpg stanza contains signature information for GPG-signed distributions. The form is:

gpg <signature>, <parameter>: <value>

where <parameter> is one of key_id or key_url, and <signature> points to the detached signature of the distribution. Commonly, the signature follows the url value. Example: libreoffice.rb.