their placement can make or break a defense sometimes what would be the ideal placement for gaussies and hangars defending a homeworld ??
Hangars should be anywhere, really. The fighters/bombers are fast enough to get to most places. The hangars themselves, though, should be within range of a repair platform (which is more important than the defenses themselves, imho).
The Gauss cannons and the other types (platforms from here on out), though, should be set up near jump points. Meaning, the enemy can only come into your gravity well at certain areas along the phase lanes. They can't jump to the backside of your planet. So, placing your platforms where the enemy is likely to jump in makes the most sense. Obviously, you want some overlap in their firing range to maximize damage.
I prefer to put mine pretty close to the planet and (if I can) within the range of a repair platform. The reason I put them close to the planet is that ships moving inward in a gravity well move faster then when they try to leave a gravity well. So, if you have them pretty far out, they don't get to do as much damage. I let my fighters take care of the far reaches.
Also, keep in mind that pirates go straight for trade and refinery platforms. I use them as bait to run straight into my defenses.