Both of those are pseudo-pets, and they work differently so you can't really compare them.
My understanding is that Volcanic Gasses is a pet that then spawns a total of 6 other geyser pets. Each of the pets has a chance for the proc to hit. (I personally have not tested this . . . this is how it has been discussed on these forums.) So it has lots of chances for the proc to hit.
Poison Gas Trap isn't a controller power. If you are talking about the Defender version, I suspect that the proc can only go off when the trap is set off. But again, I haven't tested it. I know that the Lockdown +2 Mag proc is often suggested for Poison Gas Trap.
Earthquake is different because it has a chance to go off when the power is first cast and then every 10 seconds thereafter.
Is it worthwhile slotting the damage proc in Volcanic Gasses? After you have slotted up enough hold and recharge, it probably is worth slotting if you are trying to get some damage in your build. The purple Unbreakable Constraint Chance for Smashing Damage is worth slotting.
My Earth/Rad that I based my guide on was leveled up before IOs, and I haven't gone back to fully IO him out . . . sadly, he is on a server where we don't play all that often anymore. I have an Earth/Storm on my main server, but I haven't taken the time to IO her out yet . . . working on too many others.
LOCAL MAN! The most famous hero of all. There are more newspaper stories about me than anyone else. "Local Man wins Medal of Honor." "Local Man opens Animal Shelter." "Local Man Charged with..." (Um, forget about that one.)
Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
How do procs like "Ghost Widow's Embrace: chance for psionic damage" work in a non-targeted AOE hold, like Volcanic Gases or Poison Gas Trap?
Is it different for something like "Explosive Strike: Chance for Smashing Damage" in Earthquake?
What's the consensus on whether it's worth using these procs with these sorts of AOE controls?