As others have pointed out, to trace the source to a restaurant, you need multiple reports from people who ate at the same time.

A big reason why is that most people have huge misconceptions about the onset time of food poisoning symptoms relative to ingestion. Check out the tables for symptom onset time from the Mayo Clinic and the US FDA.

While symptoms from ingesting Bacillus cereus typically appear within 10-16 hours of ingesting the affected food, symptoms from Campylobacter jejuni don't appear for 2-5 days after ingestion, and symptoms from Giardia lamblia don't appear for 1-2 weeks. Salmonella takes 1-3 days to manifest, etc. etc.

While it's possible that the sausage gravy caused your symptoms, it's also quite possible that you were experiencing symptoms from something you ate days (or even weeks) earlier. Most people don't realize that there are often fairly long reaction times, and thus blame something that wasn't really the culprit.