Can I can cast dispel magic if someone drink this potion. It says that you gain effect of a gaseous form spell. Does it count as spell yes or no?

At my table, I would say no. It says you gain the effect of gaseous form, not that the potion casts the actual spell on the person drinking it. It's a very subtle distinction, but an important one. 

I would say that absolutely, yes you can. Unless someone at the table could justify to me that this potion is somehow not an effect produced by magic, then I would say that the spell dispel magic -- in that it dispels magic -- would be effective against a magic potion that is producing a magic effect on its imbiber. Dispel magic's spell description even specifically states that it dispels magical effects in addition to spells. 

The only exception to this in my mind would be a potion that takes instantaneous effect and thereafter has no more presence on its user -- like a healing potion. You obviously couldn't dispel that. As to whether or not you could cast a counterspell on it? Well, that I'd say depends on the table.

Main ingredient of Paladin Grenade

Can you still take damage while in gaseous form? cast spells? etc?

Several months later, and inquiring minds want to know.  I would say you can take damage just as if attacking a ghost or something.  Still not clear if you can cast spells.  You don't need to hold concentration, sooo...yes?

You can't cast spells nor make any attack, as it is written in the gaseous spell discription.