So sorry to hear of her passing.
Unfortunately chickens are very good at hiding illness and can degrade quite quickly. At the point you mentioned she was having difficulty with falling over, a hen is generally pretty far gone at that point.
Might be a good time to give everyone a good check-up. Weigh them or check their condition by feeling the breast around the keel bone. It should not be prominent, though may be less developed on hybrid breeds. Note anyone that seems lighter than everyone else or doesnt seem bright, alert and responsive.
I would worry about a possible viral cause for that gurgling, so keep an eye out for that symptom in your other hens.
If it was sour crop, this is caused by a yeast infection - typically, a slow emptying crop will cause, be caused by, or perpetuate this issue.
The crop may feel swollen or doughy, may contain air, may be visible in the back of the throat as a white growth (or by the vent), may smell yeasty or like a sourdough starter.
It will be accompanied by a lack of appetite and, as mentioned, slow crop emptying.
Ive had a hen get sour crop due to slowed digestion and seen it in wild kereru, but have never heard a bird gurgle from it. Hopefully, its not something that will run through your flock. Be vigilant and seek veterinary diagnosis if you start seeing symptoms in other birds.