Comes from the once semi-popular cultural slang term "its all gravy" or "its all good", or more recently "no worries." Simply put it means "its OK." Its all gravy in the navy" is simply an addition...
It means that which has been easily acquired, or something that was extra in some way, such as a bonus. Much in the same way that gravy is an extra on top of the basics of a meal. A "gravy train" was...
I dont know what you mean but maybe the person was hungry. Another suggestion: Back in the 1960s the term 'dig' was used informally to indicate one liked something, as in 'I really dig this music'....