If you look at the Ubuntu boot sequence, while gdm isn't the last thing
that we start, it's not far off. Very few things are started after gdm,
and this is mostly because X takes quite a while to start so we have some
"free time" here.
We deliberately don't keep starting services throughout login, because as
you note, I do all my timings until full desktop login which is when the
computer is actually usable - not just to when the gdm screen is up.