Gun sights and aiming are intimately connected. Most people do not need to really care about gun sights, although a solo or a gunsmith should find the following knowledge important.
You can have as many gun sights on your gun as you manage to stick on it, but only one sight can be used per turn.
Sights give you their WA automatically. If you take the aim action you gain the aim bonus of the sight. If the sight gives a bonus as free aim you can gain this bonus as a minor action.
Iron sight: usually + 2 with Aim Action, see Iron Sight article for more details
Laser sight: + 1 as free Aim, + 2 with Aim Action
Telescopic sight: + 4 at medium or long/+5 at extreme with Aim Action
Reflex Sight: reduces penalties by one for Turning to face target, Snapshot and Changing Targets (and similar) as free Aim
Targeting Computer: + 5 as Aim Action (target is selected, gun locks trigger until gun is correctly aligned) if the computer can not point the gun; as free Aim if computer controls the gun
Close Combat Sight: attack as normal at Point Blank Range, but shots that “missed” do not use up ammunition because they were never fired in the first place
Cybernetic (requires a neural processor)
Cybernetic link: + 1 to weapon accuracy
Targeting Scope: + 1/scope option as free Aim (only once per optic)
Cyberscope: + 3 at medium or long/ + 4 at extreme as free Aim; Also functions as a regular telescopic sight