Why numerical superiority compounds in a gunfight

How effective was it for several ships to gang up on a smaller enemy force before radar fire control?

Very effective, because the outnumbered ships had to divide their attention while each attacker could concentrate on one target.

In a six-against-three engagement, the smaller force must either leave half its opponents unengaged or split each ship's fire between two targets. The larger force can give every ship a single target and concentrate two ships against each opponent. Ignored attackers can fire without interference from return fire. Dividing fire instead reduces the concentration brought against any one target and the chance of inflicting critical damage. Numerical superiority therefore creates an additional targeting advantage.