gpg-agent not found for homebrew - homebrew

Question

I'm attempting to install gpg-agent, but I am getting an error that the formula doesn't exist:
$ brew install gpg-agent
Error: No available formula with the name "gpg-agent"
==> Searching for a previously deleted formula (in the last month)...
Warning: homebrew/core is shallow clone. To get complete history run:
git -C "$(brew --repo homebrew/core)" fetch --unshallow
Error: No previously deleted formula found.
==> Searching for similarly named formulae...
Error: No similarly named formulae found.
==> Searching taps...
==> Searching taps on GitHub...
Error: No formulae found in taps.
However, it appears that this formula is still active: https://formulae.brew.sh/formula/gpg-agent
Is there something I have configured incorrectly? Other formulas are working as expected (e.g. brew install gpg, brew install pinentry-mac).
In case this information is helpful:
$ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!
Warning: Unbrewed dylibs were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected dylibs:
/usr/local/lib/libtcl8.6.dylib
/usr/local/lib/libtk8.6.dylib
Warning: Unbrewed header files were found in /usr/local/include.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected header files:
/usr/local/include/fakemysql.h
/usr/local/include/fakepq.h
/usr/local/include/fakesql.h
/usr/local/include/itcl.h
/usr/local/include/itcl2TclOO.h
/usr/local/include/itclDecls.h
/usr/local/include/itclInt.h
/usr/local/include/itclIntDecls.h
/usr/local/include/itclMigrate2TclCore.h
/usr/local/include/itclTclIntStubsFcn.h
/usr/local/include/mysqlStubs.h
/usr/local/include/odbcStubs.h
/usr/local/include/pqStubs.h
/usr/local/include/tcl.h
/usr/local/include/tclDecls.h
/usr/local/include/tclOO.h
/usr/local/include/tclOODecls.h
/usr/local/include/tclPlatDecls.h
/usr/local/include/tclThread.h
/usr/local/include/tclTomMath.h
/usr/local/include/tclTomMathDecls.h
/usr/local/include/tdbc.h
/usr/local/include/tdbcDecls.h
/usr/local/include/tdbcInt.h
/usr/local/include/tk.h
/usr/local/include/tkDecls.h
/usr/local/include/tkPlatDecls.h
Warning: Unbrewed .pc files were found in /usr/local/lib/pkgconfig.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected .pc files:
/usr/local/lib/pkgconfig/tcl.pc
/usr/local/lib/pkgconfig/tk.pc
Warning: Unbrewed static libraries were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected static libraries:
/usr/local/lib/libtclstub8.6.a
/usr/local/lib/libtkstub8.6.a
Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run `brew link` on these:
nmap
Warning: Homebrew's sbin was not found in your PATH but you have installed
formulae that put executables in /usr/local/sbin.
Consider setting the PATH for example like so
echo 'export PATH="/usr/local/sbin:$PATH"' >> ~/.bash_profile
$ brew config
HOMEBREW_VERSION: 1.7.5
ORIGIN: https://github.com/Homebrew/brew
HEAD: d639559c4de48faad79d3ecb1ec84dc851ea66f2
Last commit: 3 days ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: bd29bc2b97641173dbe4e5ec2a6fe484dfa9dd93
Core tap last commit: 2 hours ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_GIT: git
CPU: octa-core 64-bit kabylake
Homebrew Ruby: 2.3.7 => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Clang: 10.0 build 1000
Git: 2.19.0 => /usr/local/bin/git
Curl: 7.54.0 => /usr/bin/curl
Java: N/A
macOS: 10.13.6-x86_64
CLT: 9.4.0.0.1.1526532315
Xcode: 10.0
XQuartz: N/A

Answer 1

The gpg-agent formula was removed by commit 965e1. The reason is given in the commit message:
commit 965e130e04e5900e35bf1f0b6ebad9d1c2f680a7
Author: Dominyk Tiller <dominyktiller#gmail.com>
Date: Sun Aug 19 23:07:17 2018 +0100
gpg-agent: delete
Is a redundant formula without `gnupg#2.0` around; `gnupg` >2.1 ships
its own `gpg-agent`.
So if you want gpg-agent install gnupg.