She had a fecal test no gapeworm ova found. I took a throat swab and sent it to the lab and she has a virus called Infectious Laryngotracheitis Virus.
She is quite poorly but I think may be beginning to be on the mend. Definitely not out of the woods - she has lost a lot of weight and is very weak but looks a bit brighter in the last 24 hours.
The lab I sent the swab to uses PCR to test for mycoplasma (which is what I was guessing), Marek's (because why not) and this ILV thing that she turned out to have.
In case you think it may help you to know more this is their website:
https://www.vetdna.com/
It turns on that 'gaping' is what chickens do when they have trouble breathing or have a sore throat - it is not specific to gapeworm.
Right now I am still focused on trying to nurse Bella back to health - but at some point I will face whether or not to vaccinate the other chickens and any newcomers. The mystery here is that the others haven't gone down with it (at least not so far).
I do hope your poor hen makes a speedy recovery - it is tough seeing them struggle like that.