Gilas do spend the majority of their lives underground in the wild, and in captivity they are usually not too active as adults. When they are active, it is during the night hours and consists of digging. How much you see them depends on how the cage is set up and what hide cover is provided.
I do not know of any dedicated tests for captive life spans and behavior, but there have been many heloderms in captivity that have survived over 25 years!
It is conceivable that these animals may enjoy a longer life span in captivity than in the wild, since their natural preadtors and dangers are removed.