I think the GK Grandmaster will have definite reasons to take the new Libarious Powers, since they provide so much utility to a Grey Knight army. Here is my look at each power from this perspective:
Primaris — (WC1) S5 AP3 Blast — Solid little AP3 witchfire which can help against MEQ opponents or other medium/light infantry.
1 — (WC2) Blessing that allows re-roll of saving throws for Psyker and his unit — AMAZING power that many people have noticed since the Angels of Death supplement came out… makes everything MUCH more survivable (affects armor, cover, and Invul saves). This works particularly well when the Grandmaster is with a unit of GK Terminators or Paladins, since statistically this kicks them up to a re-rollable 2+ save against non-AP2/1 weapons and better than a 4++ Invul against AP2/1 attacks. It can even work with Power Armor GK units, improving their 3+ save measurably and also enhancing any cover saves they have access to by a significant degree.
2 — (WC1) S6 AP4 20″ Beam with Pinning — Pretty solid, especially against hordes and light vehicles (with 20″ range, could damage multiple Rhinos or Chimeras in a single shot). This can be the equalizer for GKs when having to deal with lots of weak enemy units that outnumber them.
3 — (WC1) Blessing that gives Psyker Fearless/Adamantium Will, plus a 12″ bubble of 4++ against enemy Witchfires — Not the greatest for GKs, but combines well with Brotherhood of Psykers and The Aegis to protect against enemy psychic powers, especially Eldar Seer Councils, enemy SM Libarious Conclaves, Chaos Daemon warp shenanigans, and even some Tyranids witchfire attacks.
4 — (WC1) Blessing that gives Psyker +2 S/T/I/A — Speaks for itself… imagine your Grandmaster with S9 AP3 Shred/Instant death (S4 + Soul Glaive Relic + Hammerhand + this power), T6, I7, and 6A on the charge (gives old Mephiston a run for his money!)
5 — (WC1) Malediction that causes Psyker and an enemy Psyker to roll off (2D6 +PML for friendly) vs (only D6 + PML for enemy); if tie or friendly wins, enemy loses 1W (no saves) and, if win by a lot, enemy also loses a power — Great power against enemy Psyker armies, though a little situational in general gaming.
6 — (WC2) Malediction that reduces enemy Invul by -2 (to a minimum of 6++) — AMAZING power that is a game-changer; imagine Necron Wraiths that only have a 5++, or Tau Stormsurges whose Shield Generator only gives a 6++, or TWC whose Storm Shields only give a 5++, or Archons whose Shadowfield only gives a 4++, or Tzeentch Daemons with Grimoire that are reduced back to a 4++… you get the point! This power can be the one which wins or loses games, especially against many deathstar type units.