A word that means 'gnashing' but related to 'gums'

Here's a sentence I made up.

"The zombies stumbled towards her ____ing their toothless gums."

I can't use 'gnashing' because of its definition.

gnash

/naʃ/

verb

gerund or present participle: gnashing

grind (one's teeth) together

Google Dictionary

So what were the zombies doing?

I think the normal term is "champing".

Generally, far as I can tell, champing is where the mouth works and chews at either nothing, or at a specified something (chewing tobacco, gum, cigar, bridle-bit...) that will not be damaged or destroyed.

Unfortunately, while I can find quotes, I can't get a dictionary reference which doesn't include teeth, though the canonical "champing at the bit" clearly doesn't involve teeth, since a horse's bit goes into the mouth behind the horse's teeth.

he smacked his lips and champed his gums while muttering

-- The Scarlet Plague, by Jack London, 1915.

she was an old hag in a great tremendous poke-bonnet, ambling along backroads at twilight and champing her gums and spitting at people she passed.

-- God Bless the Devil, by James R Aswell, 1940.

A tropical field rat, gnawing a hole in a cardboard box, recently scampered off with Boardman's lower set of false teeth. "I'm in an awful fixth," the Seabee explained as he chomped-champed his gums. "What good will my uppersth be without th' lowersth?

-- Marine Corps Chevron, by Sgt. A. D. Hawkins, 25 Nov 1944

I gave only one look at the aged grandmother: wrinkled, bony, hunched, almost bald, toothlessly champing her granulated lips, her eyes red and gummy"

-- The Journeyer, by Gary Jennings, 1984.

Mother is there and eating well -- I know that, having seen her at it -- greedily sucking in her thin ragged lips, champing toothlessly away, her eyes dull and lustreless

-- The Time: Night, by Li͡udmila Petrushevskai͡a, Sally Laird, 1992.

an old mottled Chinese nonya champed her gums at the open door

-- The Long Day Wanes: A Malayan Trilogy, by Anthony Burgess, 1992.

The crone champed her gums together and nodded brusquely

-- Forged by Fire, by Janine Cross, 2008.

a toothless middle-aged lady whose wide mouth seemed to be the only mobile part of her enormous face. She had a habit of champing their lips while she sat silent

-- Dance Of The Apprentices, by Edward Gaitens, 2010.

The old woman suddenly lurched, champing her gums irritably.

-- Reading the Sauce Bottle, by Martin Stuart, 2013.

Frankly, I'd stick with gnashing. It is true that the standard meaning refers to teeth, but "gnashing their gums" makes the point that they are using their mouths as if they had teeth, and seems a reasonable literary device.

'Mashing' one's gums is a usage I've come across, but it's rare:

The things they do sometimes stick in your mind forever – like when my [niece] was little and on a train with a very elderly man who was mashing his gums ...

[Don't mess around with my Thermomix!!]