Spoken like one who is scared. They are not the end all be all of codexes. A normal MEQ army will be able to fair well against them, the player will just need to play well. As for HTH armies they also can do well but need to be much smarter on what they attack and get into.
Overall the dex hassome good things but also some glaringly bad things. So its a matter of if the general can overcome the them and take the day.
Spoken like someone who has truely taken a huge chug of the GW Codex Koolaide.
If you seriously think a normal MEQ army will do well against the GK, you are in need of some serious help. The entire army has Power Weapons, which changes the ENTIRE CLOSE COMBAT DYNAMIC. People complained about the Sanguinary Priest giving anyone in 6 inches FNP and FC, and then realized they were a single wound T4 model with no invulnerable save and could be picked out in close combat.
Now I know you are going to bring out the "Well they only have maximum of 24 inches on their weapons". Mounted your Psycannons have a 30 inch range and 2 shots a peice. If your rhino does not move you have 4 shots with a Str 7 ap4 RENDING weapon. This would be a minor issue if you had no way to get a unit near my deployment zone before the 2nd turn, however you will be seeing that "shunt" move teleporting a unit of somekind(Dreadknight, GKSS) within enough range to assault the next round.
I dont think teleporting is OP btw, esp since you cannot do it as a scout move(which some internet superstars still think is possible) and I think it adds a new element to the game which is fantastic.
All in All, the codex would be 100% perfect if the basic trooper was not given a freaking power weapon. This is what I feel is the most ridiculous portion of the codex. Everything else can be planned for, but when the shift in Close Combat is this huge, something is amiss.