gdal-abi-2-1-2 software package provides virtual package provided by libgdal20, 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 gdal-abi-2-1-2
gdal-abi-2-1-2 is installed in your system.
Make ensure the gdal-abi-2-1-2 package were installed using the commands given below,
$ sudo dpkg-query -l | grep gdal-abi-2-1-2*
You will get with gdal-abi-2-1-2 package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | gdal-abi-2-1-2 |
| Version | |
| Description | virtual package provided by libgdal20 |
| Sub Section | Virtual |