Ordinary criminal gangs might intimidate people as a consequence of their actions but it is not typically the main purpose of the illegal activity. eg. If a gang member sticks a gun in your face and demands your wallet, it is usually because they want your wallet rather than to influence your political speech or how you vote. I think once you reach a certain scale of criminal activity that line can become blurred though.
The KKK effectively was a terrorist group and was treated as such, even if it didn't get the label back then. They were using violence to intimidate for political purposes. The President was given the power to go after them with the military where local law enforcement couldn't or wouldn't deal with them. Civil rights era KKK was treated in much the same way this executive order seeks to treat Antifa: by siccing federal law enforcement on them. If they were still as active as Antifa is now, and if local law enforcement was unable or unwilling to deal with it, they may well be labelled a terrorist organization.