I haven't really done a lot of combat so most of what I say is either what I believe I heard or complete guesses. However, I'm pretty sure I'm right, but if I'm not I'm sure someone else will come and correct me.
I have some basic research done in gauss cannons and lasers and have turrent gear of 5000km, base fire control range of I think 24k and base tracking speed of 4000km. I think I have ROF 3 and launch speed 3 for the Gauss but having had a look at some designs these things seem woefully short of range. Lasers are up to 15, capacitor 3 and near ultra-violet.
Gauss has it's ups and downs. See
this old (but I'm pretty sure it's still up to date) thread. Mainly the bottom of the first post
My first few goes at building a turreted defence system to cover the fleet look horrible. My best fire control at four times size is well under the range covered by most incomming missiles on a 5 second tick and is almost as big as the turrets themselves.
Set them on Final Defensive Fire PD mode. This allows any beam weapon (including the extremely short ranged Gauss) to get a single volley at each group of missiles no matter how fast they are going. However, they will only shoot if the missiles are going to hit them, and thus will not cover an area and will not cover ships outside of their TG. Fortunately, they will still cover other ships in their own TG.