gdb-source software package provides GNU Debugger (source), 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 gdb-source
gdb-source is installed in your system.
Make ensure the gdb-source package were installed using the commands given below,
$ sudo dpkg-query -l | grep gdb-source *
You will get with gdb-source package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Development |
| Package | gdb-source |
| Version | 7.11.1-2 |
| Description | GNU Debugger (source) |
| Sub Section | devel |