I am of the opinion that most
TAC (Take-All-Comer's) lists are "tailored" to a certain degree to the current "meta" rather than any particular opposing army. That is, we all have expectations in a tournament setting of what we think we will most likely encounter and how we think the game is played competitively. And if you want to be more specific, what we think we will most likely encounter and how we think the game is played competitively
in our local area. Right now, that meta tends to be mechanization and predominant tournament army the grey knights. Now I'm not saying that one should build his list specifically to beat the grey knights, but one does need to take into consideration how he will be able to handle them when building his list.
Necrons are another army that has changed the current meta. As it is the most recent codex, a lot of people have started playing the new necrons. Thus, when you bring a
TAC army to a tournament, you have to consider the possibility that you will face them. As such, in competitive play, I consider searchlights as mandatory nowadays on Imperial vehicles and to a lesser degree, dozer blades (only if the points allow). However, if you know that there are hardly any necron players in your local meta, then you can "tailor" your list by actually not bringing searchlights and dozers to your lists. Again, this is a form of tailoring because you know or can expect what type of armies you will or won't see in your local area. However, at a larger tournament - say a
GT like Adepticon, NovaOpen or the BayAreaOpen - you really don't know what type of armies you will be facing. Therefore, you need to bring a well-rounded
TAC list that can handle most army builds (I didn't say all because I don't think there is such a build). In such a list, searchlights are just too incredibly cheap not to consider.
Anyways, getting back to the topic, can necrons handle the grey knights? I believe they can, though like everyone else, the necrons will have a tough time against the knights. Most armies do. However, I don't believe you need to make a list specifically to counter the grey knights. Rather, you should build as well-rounded a necron list as you can because if you build a list specifically to fight another list, what happens when you don't face that type of list? You need to spend the time, money and resources to build another list to take on another build. More importantly, you don't "grow" as a player. If you want to mature as a player (i.e. become a better player), you need to learn how to play against all types of armies with what you've got, so it's better
IMO to make what you've got the best that it can be. In other words, a Take-All-Comer's list.
For my necron list, I actually play a fast, assaulty
TAC necron list. I call it the Maximum Threat Overload (MTO) necrons. It is based on the premise of overloading your opponent with more threats than they can handle and this strategy can be applied to almost any opponent. So far, I've achieved moderate success against the grey knights, though I admit that I haven't really played against them too much.
Anyways, here are 2 of my battle reports against the grey knights:
The Grey Knight Challenge Part I - 2K MTO Necrons vs Draigowing!
The Grey Knight Challenge Part II - 2K MTO Necrons vs Crowe-Purifiers
And here is a report of my wraithwing necrons (a necron list using mainly wraiths as their primary offense) against the knights:
The Resiliency of Wraithwing - 2K Games vs Grey Knights and Deathwing
By no means are these lists the only ways to play against the grey knights. Rather, they are a play-style that I enjoy playing. I think that is something you also need to consider when creating your list - what type of style you would like to play.