... Okay,  but is the gas flammable?

No, it would have to specifically state that it is flammable.  Likewise, it will not displace enough air to cause flames to be extinguished. 

You can always create your own version that is flammable; though I imagine if it does both poison and potentially fire damage, then it should probably be a 1st level spell instead of a cantrip. 

So are you Poisoned after you get hit with the poison spray?

No

Depends on the poison?

Does the poison spray disappear after you use it?

If you cast this and there are 2 enemies standing directly in front of you at 5ft and 10 ft away, does it hit them both? 

It is a single target spell. The spell will hit whatever creature you target, even if the target is 10 ft away and there is another creature between you and it. I think you can think of it as a burst of poison gas blasting in a tight stream, past the first creature and at the second.

If I'm wizard is my dc save = 8 +  your proficiency bonus + your Intelligence modifier + Con. Mod?

...Now,let me get this right, i could use this as the gas that hurt Katniss and her crew in the 75th hunger games.?

No, your wizard spell save DC is just 8 + proficiency bonus + INT mod. The target of this spell makes a CON save (d20 + proficiency bonus if they are proficient in CON saves + CON mod) and must meet or exceed your spell save DC in order to avoid the effects of the spell.

thank you

Depends on the gas

Aka big brother buttercup

Incorrect, it does not "depend on the gas".

Casting word is:

Toxus Wasterium

Why is poison spray not in the PHB? And if it is can someone point it out to me please?

Can poison spray linger since it is in gas form?

Quote from Spellbreaker3 >>

Why is poison spray not in the PHB? And if it is can someone point it out to me please?

It is. https://www.dndbeyond.com/sources/phb/spells#PoisonSpray

Quote from JoeyW1071 >>

Can poison spray linger since it is in gas form?

The duration is "Instantaneous", and the description says nothing about ongoing effects, so no. The target just takes damage once when you cast the spell if they fail their save. You could imagine the magically created gas is magically guided into the target's face, and then magically fades away as quickly as it was created.

Quote from Spellbreaker3 >>

Why is poison spray not in the PHB? And if it is can someone point it out to me please?

 Page 266 of the PHB