You know, it's interesting. After a lifetime of outdoor activity, military experience, in and assisting auto/motorcycle accidents, "gross" has little effect on me anymore. What many would find upsetting, I find are just part of the journey. Found a human femur once, buried dentures, buried animals, decomposing animals of all sorts, rotting whales and other sea creatures. Unpleasant smells yes, gross, not so much.
But, to contribute, the worst smell, we were salvaging an abandoned boat. Onboard was a cooler. You have smelled nothing as bad as a cooler full of festering, 85 degree squid baits fermenting in their own juices.
I also worked for Roto Rooter (sewer cleaning) for about 5 years. Maybe that's when I lost my "that's gross" sensation.