I want to install QGIS today and run into following issue:
qgis : Depends: gdal-abi-2-3-0 but it is not installable
It is not installable because it does not exist in buster, only in sid. It is a virtual package for libgdal20.
So I installed libgdal20 and crate a fake packed.
$ cat gdal_abi.txt
Section: misc
Priority: optional
Standards-Version: 3.9.2
Package: gdal-abi-2-3-0
Version: 2.3.0
Depends: libgdal20
Description: fake package for qgis which needs a gdal-abi-2-3-0
# apt-get install equivs
$ equivs-build gdal_abi.txt
# dkpg -i gdal-abi-2-3-0_2.3.0_all.deb
All fine :)