brew tap domt4/homebrew-gpg
And then updates will be picked up in the normal way:
brew update
If the "experimental" label isn't indicative enough of this, please do not treat this command like it is providing you any additional security.
This originated with a project last year that was never quite pushed through to completion for a variety of reasons, but my intention wasn't to abandon the idea & the desire remains to see it become something that ships with Homebrew.
Usage is pretty basic.
brew fetch-gpg <formula> <URL to GPG signature>
such as:
brew fetch-gpg curl https://curl.haxx.se/download/curl-7.55.1.tar.bz2.asc