Gas Arrow Deadeye - mini guide

#Update
The build is getting nerfed in the short term but depending on how the "potency of the Ignite or Detonator contributes to determining the damage of the Gas Cloud explosion" part plays out, it might not be too bad in the end.
The explosion itself is fire damage, which can ignite, so each explosion should be stronger than the last as the ignite > hit > ignite loop ramps up.
Alternatively, we can just build for as much flat fire as possible and try to ignite with the initial hit of the arrow.
I wouldn't panic just yet, but know that the build will be weaker after the first hotfix, and we'll have to wait and see how the second fix plays out.

Guide

I'm sure you've seen some of the clips floating around of gas arrow/grenade totally demolishing stuff, but I haven't seen anyone post much of an actual guide yet.

The basic premise is that gas arrow creates a cloud of gas that explodes on contact with an ignited enemy or detonator skill.
This is quite easy to do with just a level 1 flame wall - just drop the wall on top of the gas cloud and when an enemy touches the fire, the whole thing explodes. Later on you can find other means of detonation that don't require casting a secondary spell.

The explosion is an aoe attack with 100% phys to fire conversion, scales with projectile damage and can "shotgun" with overlapping aoes. This gives us several good angles for scaling.

#Passives
My tree looks something like this

Its pretty much all damage, but you're in range of plenty of evasion nodes if you want to grab more defense.
Stat-wise, I basically just get enough dex to equip my armor and gems, and the rest goes into strength. I only needed 1 point into int to use flame wall.
#Leveling
Gas Arrow is a level 22 skill, so I just used Lightning Arrow and Lightning Rod until then. You can use whatever strikes your fancy - you only need to get to mid act 2. #Gems Gas Arrow + Scattershot + Fire Infusion + Primal Armament + Fire Penetration + Elemental Focus Flame Wall
Barrage + Perpetual Charge
Herald of Ash + Vitality
Wind Dancer

Obviously you're not going to have a 6link gem starting off, but that's the order I prioritized for my supports.
Barrage gives a nice burst of damage at the start of a boss fight. I wanted to use Combat frenzy as well, but it doesn't synergize with this build well since it requires freeze, electrocute, or pin to generate charges.

You can also look into something like infernal legion instead of flame wall.
#Ascendancy Deadeye, Point blank + extra arrow, and the tailwind branch.
As noted above, my initial plan was to use barrage and frenzy charges but the build doesn't have a reliable way to generate them.
#Gear Weapon: Any bow with good physical and/or fire damage.
Quiver: Also anything with good physical and/or fire damage.
Armor: Evasion gear with defensive stats.
Jewelry: Your choice, either more defense or you can get more flat damage here.

Uniques

I'm currently using Death's Harp and Radiant Grief, but neither are mandatory.

Death's harp has very low base damage, but +3 projectiles which more than makes up for it. I just supplement it with added damage on my other equipment, like quiver, rings, and gloves. Because of the projectiles on the bow, we can use another damage support instead of Scattershot. Later on I'll ideally be looking for a dualstring bow with an additional projectile (+2 total) and good phys/fire rolls.

The helmet is a total game changer since it automatically ignites nearby enemies. This means no need for flame wall or other means of manually igniting enemies. The range isn't huge, but I personally like that because if I fire gas clouds ahead as I'm running, it gives them a little time to expand fore more aoe before they detonate.