I played all the fallout series from the start and usually played wtih characters with small gubs skills (guns for FNV). In FNV my second char is a weird one with 1perception and 1 charisma. I made her a unarmed char and never spent a exp point on guns skill. Char's gun skill is 16 and she can use the guns perfectly, stealth criticals are instant kill most of the time. Only and not so important downside is you can not "focus" very well but it does not affect the game so much, i can snipe the opponents very well. My qeustion; What is the formula of skills effect on gameplay? (not the initial formula). And why i should spent points on gun skill other than passing the skil checks through dialoge?

Because as it says in the game, you won't be as accurate with a gun if you don't meet the skill requirement...? I also think you won't do as much DAM with it, but I could be wrong on that one.

Damage penalty for not having 100, roll for spread skew to min spread rather than max spread; and if skill doesn't meet requirement sway is larger

At 0 weapons skill, you do 50% damage with a fully repaired weapon, increasing by apprximately half a percent per point of skill. For guns, higher skill also reduces wobble and spread, also increases VATS accuracy. For Melee and Unarmed, it unlocks special attacks.