They both mean the same thing, but the usage can be slightly different. In your example, you could say that the worms were both "disgusting" and "gross". You can also use both words to describe behavior. For example, if you saw someone picking his nose, you could say either "gross" or "disgusting" (or both). The difference is that "gross" is a little more slangy than "disgusting". You would be more likely to hear a child or teenager use the word "gross" than an adult. Be aware that "gross" also has other meanings: 1. A "gross" is a number equal to 144, or a dozen dozen (12x12). 2. It can also mean the same thing as "total", as in, "the total amount". Ex. "The gross profit was $1000," which is another way of saying "the total profit was $1000."

In that particular sense they mean the same. 'gross' is rather an informal exclamation of disgust. As sanzziosity says 'gross' is a slang term, while 'disgusting' is formal as well.