hate to intrude, but a CIWS isn't just a gauss cannon. It's an entire system (e. g FCS, ECCM, Sensor, tracking gear etc) specifically designed to attack high-speed objects at close range to prevent damage to the vessel. It's a last resort defence attempt after all your other defensive options have failed. Increasing the range on it would mean it could engage targets earlier, yes-- but a larger range would mean it would have more targets to choose from, potentially causing a process logjam in the FCS, engine/gear damage from overuse, dead batteries, overheating etc (although overheating a gauss cannon would be pretty hard-- the main heating problem would be the capacitors. )
If you want a gauss cannon to attack something else just create a turreted one. the range will reach out further with research and technically it will have a higher accuracy rating, depending on the size you make it. They're a great weapon for fighters or FACs, same as railguns. The only problem I have with Gauss weapons is the size. There's nothing really stopping you from making a man-portable solenoid gun, so the accuracy changes with reduced size really don't make sense to me.
In short you should only use CIWS for a last ditch defence.
1) Launch AMMs to intercept, ideally a minimum of two dependant on enemy missile speed. Against missiles marginally faster than yours it would probably take three AMMs or more.
2) Any surviving incoming missiles are taken out by long-ranged beams, or alternatively a swarm of fighters/FACs . I find my gauss and railgun fighters actually manage to gun down missiles heading for my big ships pretty often before they rush to swam my enemies.
3) Multiple FCS on beamships-- generally my area defence ships always have several weapons always set to area defence, and several always set to final fire.
just my 2 cents.