gnupg-agent software package provides GNU privacy guard - cryptographic agent, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install gnupg-agent
gnupg-agent is installed in your system.
Make ensure the gnupg-agent package were installed using the commands given below,
$ sudo dpkg-query -l | grep gnupg-agent *
You will get with gnupg-agent package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Utilities |
| Package | gnupg-agent |
| Version | 2.1.15-4 |
| Description | GNU privacy guard - cryptographic agent |
| Sub Section | Utils |