I have tried opening the entire output file … and it failed.
Can you be a bit more specific? Avogadro can read output files directly.
I also tried copying just the coordinates of the final geometry
You can paste this into the Cartesian Editor as well. In Avogadro 1.2, it’s under Build => Cartesian Editor. In Avogadro 1.9x it’s under Build => Atomic Coordinate Editor.
You’ll need to tweak the format once you paste, since Gaussian includes the atom index in the first column and charges (usually 0) in the 3rd column.