The check gas cap comes on if a vapor leak is detected after a leak test is performed. The test is performed during a certain drive cycle. If a small leak is detected the check engine light will come on. So if after 3 tests no leak is detected the light will go off. The test itself looks at fuel tank pressure over the certain drive cycle. If a large leak is detected the code will be some vapor leak code. Some previous units had a separate check gas cap light and a check engine light because the vapor leack test was very sensitive. Now this set up may be slightly off from today because what I am describing Was from around 15 years ago when I was at Ford power train development, but it will be similar.
Mickey & Sue Stamper
2013 GT XL 378TS
2013 Ford Escape on Landgrebe tow dolly