You're right re: the falling into a hole to die. Not scary.
The reason I asked if the gas was necessary is because I thinking about the bare necessities of what you'd need to make the SCP work (i.e. less is more).
There are ways it is out of place (if you're following the pitcher plant analogy, just thinking of real-world predators) and ways that it isn't (you have this really neat solid-liquid-gas motif, it conjures up horrible WWI-type gas attack scenarios, "sewer gas" is a thing). I would definitely go with the gas over magic concrete, as the foundation already has a magic concrete. I don't know about you, but being digested alive from the inside out is one of the most intensely unpleasant things I can think of.
Maybe you could include the gas, but try to amp up how creepy it is with a little twist detail, e.g. it leaves the victim's consciousness intact, or it begins at the extremities and works its way inward (like botulism)? Maybe you could have the network aggressively attacking, say, basements, or underground facilities with the gas? Maybe include a recovery log about a mysterious "gas leak" that wiped out an orphanage or something?
I think whether or not you include the gas should basically fall on how horrible you can make it. Which basically comes down to which horrible details you include. Is that helpful?