We've already done a pretty comprehensive review of the Grey Knight psychic powers and briefly touched upon their impact and role within the Grey Knight army. Let's look at this last part more in-depth. Now obviously any Grey Knight army is going to have a large amount of psychic powers and a certain amount of psychic defenses built into it - it's the joy of being Grey Knights that these come standard. However, the amount these basic powers and defenses are used depends a lot upon the army make up and their opponent at any given moment.
Obviously the more variety of Grey Knight units you have the more psychic power options and reliance you will have. For example, Purgation and Purifier squads are somewhat built around their psychic powers (hiding and shooting, horde control in combat) and not being able to use those powers consistently is going to be a much bigger hindrance compared to Strike or Interceptor squads and Hammerhand across the army. At the same time, Hammerhand is a lot more important against other MEQ armies than against GEQ/horde armies. Fortitude though...well that's always useful. Hard to suppress vehicles? Yes please! This is obviously only going to impact armies with Grey Knight vehicles (Chimeras don't count) but is often a huge part of the theory behind that army.
There are also a bunch of other options for psychic powers which you can access through a variety of HQ and Elites such as the Librarian and Grandmaster. These are more often army wide buffs such as Sanctuary, Psychic Communion, etc. and there is a bigger outlay in points for them. They aren't as ingrained into the army since only a few models can take them but they can really make a Grey Knight army a pain to deal with for certain opponents. For example, Shrouding is going to annoy shooting lists whilst Sanctuary is going to annoy assault armies (especially armies without grenades).
What does this all boil down to? Really not much. As a Grey Knights player where one often spends more points on the basic statlines (so basically all Grey Knight units except Paladins) against psychic defenses it can often feel like your army is floundering. You don't have access to those cheaper options such as Dreads, Preds, Speeders, Attack Bikes, Assault Marines, etc. so not being able to cast your basic powers reliably, especially Fortitude or Hammerhand, can feel like it hurts. In reality though its impact isn't as severe as it may seem, as long as you don't base your army around it. Purgation squads shouldn't be expected to always hide and shoot for example and Purifiers shouldn't be your only horde control. Running less tanks because you have Fortitude is also a recipe for disaster as well, etc. Remember, casting through most psychic defenses isn't dangerous so there's no drawback for attempting. Against some psychic defenses, most notably Runes of Warding from Eldar, you really, really have to choose when you cast your powers because of the increased chances of Perils. This is perhaps the most effective defense against Grey Knights but fortunately is pretty uncommon.
Now obviously being able to cast your powers at will makes the GK army tick along very nicely, you don't have to worry about a critical psychic test failing. For this reason targeting opponent's psychic defenses early might be a good idea, especially if your powers might be of significant use during the game. Don't go out of your way to do this however and maximising your 24" bubble is often a better way to avoid psychic defenses. Most psychic defenses revolve around a 24" bubble (sometimes smaller, sometimes larger) and keeping units out of that range allows you to use your psychic powers to their fullest potential (most notably Shrouding, Astral Aim and Fortitude).
On the flip side we have Grey Knight's own psychic defenses. Every Grey Knight unit has the Aegis which causes psychic powers to be cast at -1 Ld. Not that huge against most psychic wielders (Ld10) but it is an 8% decrease in their effectiveness. It is much better against lower Ld users such as Psychic Battle Squads but isn't going to make a huge difference. Reinforced Aegis on the other hand comes standard with all Dreadnought variants and is a -4 to cast against all units within 12" of the Dreadnought. This is huge as even Ld10 psychic wielders are dropped to Ld6 (42% success) and is a bubble radius. The key is keeping your units with the Dreadnought bubble which can be somewhat difficult with the most common setup of the Psyfledread due to differing roles (backfield compared to midfield). However, these psychic defenses are all passive and only affect powers directed at Grey Knight units.
To stop powers such as Fortune, Catalyst, Tempest, Shield of Sanguine (i.e. some of the best powers out there) etc. you're going to need a Librarian with a psychic hood. Now this isn't a huge drawback as the Librarian has access to a lot of good powers himself but unlike other Marine chapters, he isn't a simple 100 point character. More often he is 170 points + and always comes stock standard with Terminator Armor. Whilst an excellent character in his own right and the only way to gain complete psychic defense coverage (hood + reinforced aegis? ouch!) you do have to work to get him into your army. This means he's not going to be one of those easy auto-includes (unlike BA/SM/SW Librarians/Rune Priests) so using psychic powers against Grey Knights may have more success than against other lists.
Conclusion
What you need to take from this post and the one before is to ensure your list doesn't resolve around the basic Grey Knight powers. Hell, prepare yourself for not being able to always use your psychic powers (defenses or things such as RoWard) by practising games without them. If you list feels like it has had a huge drop off in efficiency you might consider doing some changes. The Grey Knight psychic powers are great and add excellent flavor to the army list as well as tactical options in game but are not something your list should be based upon. At the same time, psychic defenses come built into the army but you need to consider a lot more compared to other armies to ensure that defense is not just against offensive spells.
