This is sort of a loaded post as the combat aspect of the Grey Knight army book has been discussed to death. It's been pounded into people that Grey Knights are not an actual combat army but rather a shooting focused army. However, Grey Knights do have combat ability thanks to their Nemesis Weapons combined with their Marine statlines. Whilst this combination doesn't make them a good combat army due to their points cost (excepting Death Cult Assassins) and bog standard MEQ statline (so you pay more but die the same! rocking), how to use combat and the Nemesis weapons available to Grey Knights is obviously needed to get maximum efficiency out of your Grey Knights army.
All Grey Knight based models come standard with a Nemesis Force Sword which gives every squad the ability to ignore armor saves in combat and have the potential to instant death any target without Eternal Warrior. Add in every squad having Hammerhand which takes the Grey Knights to strength 5 and the Grey Knights have some obvious combat ability. What is important to remember with this ability however is it is secondary to the shooting aspect of most Grey Knight armies. What this ability really needs to be utilised as is back-up to the Grey Knight shooting and used to finish off squads or as a counter-attack method.
This is an important part of how Grey Knights operate on the battlefield. Having a lot of psycannnons and stormbolters allows for a very effective shooting force at 24". Since they are assault based weapons the Grey Knight force is able to move and shoot and delay combat for as long as possible but there is a limit on how long you can keep the bulk of your force away from multiple assault units. By having Nemesis weapons and the solid Marine statline assault units breaking into the Grey Knight army aren't a death sentence (unlike say Tau). Whilst their combat isn't really point efficient (they still die like Marines damnit!) it does have some punch. This allows the Grey Knight army to effectively counter attack an opponent, finish off smaller squads in combat and preserve their army structure for as long as possible.
This also allows them to be more aggressive against certain armies. Although Grey Knights don't like being in combat against units which are designed to be in combat, they are obviously better than your other shooting armies or basic units such as Tactical Marines, Fire Warriors, Guardsmen, etc. Against forces which are shooting based this gives the Grey Knight player more options in being able to deal with units in combat and play more aggressively against lists which have more firepower. It is important to remember in these situations Grey Knights still die like Marines in combat (has this been repeated enough yet) but due to their Nemesis weapons and MEQ statline, Grey Knights will out-combat some armies or unit combinations.
Ultimately Grey Knights are a shooting force but they do have some ability in combat. They have a good basic statline but pay more for their equipment such as Nemesis Force Weapons, storm bolters and the psychic power Hammerhand. This increase in points makes them much better at shooting whilst remaining mobile compared to their Marine counterparts and whilst there is an increase in combat potential, they are much more effective as a mobile shooting force. This combat potential however adds an extra dimension to the Grey Knight army in being able to launch effective counter-assaults and better other shooting or generalist units in combat. This highlights the secondary role of combat in a Grey Knights army compared to shooting but to gain maximum potential from the army, the Grey Knight combat ability must be remembered and used accordingly.
