I have the Dark Heresy: Daemon Hunter book open at the moment. The Grey Knights chapter has multiple sidebars entitled "Using Deatwatch" to help convert Grey Knights over.

On page 117, (this is in the Grey Knight Psychic Training section), the Using Deatwatch sidebar says the following:

Acquiring Psychic Powers
Grey Knights gain one Psychic ability per Psy Rating (they begin the game with two psychic powers of their choice). Only psychic powers from the Divination, Grey Knight or Telepathy Disciplines may be selected. They may not choose Codex powers.

Now this is exactly what the standard Dark Heresy rules are for Grey Knights, apparently that rule is meant to carry over. Except, the next Using Deathwatch sidebar (on page 121, in the Grey Knight Specialisations section) says this:

Librarian
Grey Knight Librarians are powerful psykers who have sharpened their psychic power to a razor-edged weapon against the Daemonic. When selecting the Librarian Specialisation, Grey Knights are not restricted in the Disciplines their psychic powers can come from. They have access to the Codex Discipline but not the Chapter Disciplines (other than Grey Knight). In addition, they obey the rules in Deatwatch for selecting psychic powers instead of gaining a Power per point of Psy Rating.

So now we have a caveat. Apparently Deathwatch Grey Knights follow the standard Dark Heresy rules unless they're Librarians. Okay, fair enough, would have been nice had this caveat been in the initial Using Deathwatch sidebar, but at least this holds together okay...

Until page 128. On page 128 we have the Grey Knight Psychic Discipline section, and on its Using Deathwatch sidebar, it says the following:

Using Deathwatch
Characters using the Deathwatch Rulebook to run a Grey Knight campaign should note that when a Grey Knight gains a point of Psy Rating he does not gain an additional psychic power. Instead, he is able to purchase one power from the Divination, Telepathy or Grey Knight Discipline at the listed Deathwatch Cost.

So now we have "everything is the same as in Dark Heresy", "everything is the same as in Dark Heresy, unless you're a Grey Knight Librarian" and "ignore the Dark Heresy rules, treat all Grey Knights like a normal Deathwatch Librarian". Normally I would expect this in a White Wolf book, not in a Fantasy Flight book.

Which ruling am I meant to use?

Yea, that confused me too. Further down on the sidebar on page 117, it says this:

"Grey Knights using the Deathwatch psychic rules should not have access to psychic powers in the Dark Heresy Rulebook."

So basically this is what it means. If you are just using the specialisations from the Daemon Hunters supplement, and the Dark Heresy Core rules to run the game, then use the first method of selecting one power per psy rating. If you use a Librarian in your game however, that shifts the system over to the deathwatch one, as stated in the sidebars on pages 121 and 128. This means that all other Grey Knights also shift over tot he Deathwatch method of purchasing powers separately.

Except not having access to the powers in the Dark Heresy rulebook doesn't actually relate to how they get more of them - just which ones they have access to.

All of the quotes I have sourced come from the guidelines for using Grey Knights in Deathwatch, not in Dark Heresy, and all of them give a different ruling on how Grey Knights gain more psychic powers in Deathwatch.

One of the quotes says that all Grey Knights using the Deathwatch system purchase their powers by spending the appropriate amount of xp, just like Librarians do. If you're not using Deathwatch, they just get one automatically every time their psy rating increases.

And one of the quotes says that all Grey Knights using the Deathwatch system gain a power per Psy Rating. And another of the quotes says that all Grey Knights using the Deathwatch system gain a power per Psy Rating unless they'e a Librarian and only Grey Knight Librarians purchase powers using XP. Go have a look at the book. All of the quotes given are about how to use Grey Knights in Deathwatch.

I think that the sidebar on 128 stating that Grey Knights need to purchase powers with xp should override the one on 117 that states Grey Knights get 1 per psy rating. I know it is unclear about it and poorly written, but I think that is how it intended to work. That's how I'm going to run it anyways.

The problem though, is it's not overriding one. It's overriding two.

Both of the other quotes are quite specific. The first says "one per psy rating", the second says "one per psy rating unless a Librarian". It's almost like those two were written by someone entirely different to the third quote.