While there's some correlation between gun control and democide, you'd have a hard time proving causation. There are other factors to consider in each of those cases.
I think yes, and no....
other factors are the determinants as to why a group forms, the shape it takes and the direction it wants to go. Other factors make the road. But it is gun control that allows them to run down anyone who gets in the way. It is gun control that prevent opposition from building a roadblock, or blowing a bridge.
Gun control is a two sided coin (or a double edged sword, if you prefer). One side is the "control" of guns in the hands of the people (registration, confiscation, etc, ultimately removal of guns from the people), and the other side of gun control is the control of guns by those in power (police, militatry, etc.). IT is the guns they control that gives them the power to do as they please, ultimately.
It is the combination of disarmament of the opposition, and control of armed supporter that allows a regeime to wield its power. Hitler,Stalin, Mao, etc., wouldn't have been able to murder millions without guns (arms) in the hands of people who would obey them, and no arms (or rediculously few) in the hands of their opposition.
Gun control alone is not the cause, but it is a necessary step for despotism to succeed. Other factors may put the dictator in the driver's seat, but it is gun control that keeps them there, and provides the horsepower for them to drive where, and over whom, they wish.