Hello, and welcome.
I'm so sorry to hear about Cosmo's diagnosis of kidney failure. Older cats are more prone to this, and he IS comparatively young, but we can never predict when this will happen. The fact that his blood work was all within normal limits last November and now the kidney disease is quite prominent is not surprising. Typically, a cat won't start showing symptoms until his kidneys are about 70% shut down.
The gulping can be an indication of nausea, caused by excess stomach acid or acid reflux and for the moment, you can ask the vet for an anti-nausea injection and an antacid safe for cats or you can give Cosmo 1/4 of a 10 mg. Pepcid AC (Famotidine) tablet every 12 hours. Here is more information about this over the counter acid controller:
The Pepcid AC is a tiny pill, so when you cut it in quarters, it's so small, you can hide it in a small amount of wet cat food or plain jarred baby food chicken containing NO onion or garlic.
With a specialized diet, medication and subcutaneous fluids, this might be treatable, but I know you will explore all options with your vet.
I hope Cosmo will be feeling better, very soon!