My point was that if you have a C5 to G5 (the 5 indication the absolute pitch, not just relative within an octave) it’s a Perfect fifth, just as if you have G5-C5. I am repeating myself but: order doesn’t matter with absolute (reads: with a certain octave specified, or: with a particular fundamental frequency) pitches!
I never said the C would be above the G and this is an assumption made by Steven (and maybe you?)
If we don’t Talk absolute pitches, you are correct: C-G is a perfect fifth and G-C is a perfect fourth, because they are complementary intervals.
Now Cb-C is a augmented unison, therefore C-Cb has to be a diminished octave, because again: complementary intervals.