Regular strike squad
GK's aren't really that bad. You just need to know how to play them effectively. For only 22-pts (assuming 10-man squads), you have a unit of marines all with power weapons and a 24" S5 assault 2 gun. The emphasis on strike squads is either anti-deepstriking defense (via combat squadding if necessary) or just shoot up the enemy until you taken you can take them out in assault. Just don't rush them headlong onto an enemy thinking that your strike squad is an uber-assault unit.
Purifiers, while good, are not necessarily a no-brainer in tournament play. I do foresee a lot of people using them, but then again, not everyone will be running a Crowe-purifier list in competitive play. As a matter of fact, you'll be seeing a lot more Coteaz-henchmen builds in competitive play than you will Crowe-purifiers.
A naked
DK isn't really much of a threat and won't really do much. It won't even attract a lot of fire initially. It isn't until you give them a personal teleporter when people start to fear them like nobody's business.