Thank you for your question.
I am so sorry to hear about Gus's sudden passing on.
If he had been coughing and hypersalivating then there are a range of conditions we do have to consider from obstruction in the throat (from foreign material, tumors, polyps, inflammation, etc) to lung based tumors (since these can be silent until a sudden critical point unlike infectious disease where we would expect signs before this) to heart conditions (which can be silent until it reaches a critical point).
To narrow down the causes, you may wish to speak to your vet about possibly checking inside your kitty. A necropsy can shed light on what caused this sudden decompensation and death, and can give you closure to what has caused Gus's situation.
I am sorry to hear that he passed away so quickly and unexpectedly.
I appreciate how shocking this can be, but do take comfort that his discomfort was very short lived with such a short period of signs. I do hope your vet will be able to help you find the reason for his passage and give you closure.
I hope this information is helpful.
Please let me know if you have any further questions.
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