Currently participating in a pretty competitive campaign. Going 3 rounds undefeated, every game is a wipe. Mortal wounds are OP. Simpy nothing can be done about them.
I use Brother Captain (more tanky) and Librarian (more deadly) a Justicar with Halberd, Combat with Hammer, Zealot with Falchions, Veteran with Incinerator, Demolisher with Incinerator, Heavy with Psilencer and Demolisher with Psilencer. Veteran with Incinerator can reach almost any target turn 1 and wipe it with high probability. I won't be taking combat with hammer outside of campaign, but it can get +1 to-hit, which is very strong. Other guys are nice, but these two are heavy lifters.
I think, brocap with staff and iron arm is perfect. 3++ in close combat with t6 makes him almost impossible to kill in first 2-3 turns, while his damage output is fromidable. There are lots of specialisms to play with. Main downside is his cost - you cannot afford more then lvl 1, because of 200-250 pts limit. But even at level 1 there are very strong specialisms.
Librarian, on the other hand, is perfect as psyker. Throwing two mortal wounds a turn with +2 to cast and +2 deny, heck yeah! He also can throw incinerators and other guys in good positions, or pull them out of combat. There are other fun options, like throwing misfortune on a terminator model. But Gate+Iron arm is probably still the best combo.
They only problem are tyranids. They are stupidly cheap. Only thing I can hope about is killing hive guards before they obliterate my entire squad.
All in all, GK in KT are way more stronger and more fun to play than in basic WH40K. I view it as a sort of compensation.