openGPG version: 2.3.6-2.2

Step:
Install package gpg2
run "systemctl --user enable gpg-agent.service"
"systemctl --user list-unit-files | grep gpg"

Expect:
Service will be enabled

Actually:
systemctl shows "Failed to enable unit: Unit file gpg-agent.service does not
exist."
and 
"systemctl --user list-unit-files | grep gpg" shows nothing.

I searched "gpg-agent" file on my disk globally and find systemd units on:
"/usr/share/doc/packages/gpg2/examples/systemd-user"
and so as to other gpg2 systemd units.

Copy these files to "/usr/lib/systemd/user" can fix my issue.
It seems maintainer of GPG2 forget to extract these files to systemd folder.