Yeah out of these GDAL is the one I use the most because it's easy to script. I honestly find it easier to use GDAL on the command line than the QGIS GUI which says a lot about the latter.
Also PROJ, the quiet bit closer to the core that does projections.
The sqlite db alone that's packaged with PROJ is a pillar of knowledge that one can only marvel at. The most authoritative and wide-ranging collection of projection/datum information I'm aware of.
GDAL should be front and center. It's the xkcd 2347 of earth observation and geographical information systems