Gravy, as an example of something yummity to put on food, has had figurative uses referring to good things for over a century. From the 1860s to the mid-twentieth century, the word was used in theatrical slang for an easy role, especially in low comedy. Another sense, a rare one, was 'a person or thing of excellence; that which is the best', which O. Henry and some other writers used soon after the turn of the century. The main senses refer to money or profit and also arose soon after the turn of the century. The still-current 'profit or benefit, especially if unexpectedly or easily obtained' is recorded by 1910. By the 1930s a subsense arose, 'money that is unearned or obtained illicitly; graft', also still in use. Gravy train is a metaphorical application of these that means 'a source or condition of excessive and especially undeserved ease, advantage, or profit; a sinecure'. The phrase ride the gravy train means 'to exploit a source of easy profit or advantage'. It is used in similar phrases as well, such as fall off the gravy train or the gravy train hasn't derailed. The construction dates from the 1910s and appears to have become popular quite quickly. Some other uses are gravy as an adjective meaning '(of a job) easy; cushy', common from World War I onwards, and gravy boat as a synonym for gravy train. http://tinyurl.com/3xf68q

I found an answer to my own question here http://www.phrases.org.uk/bulletin_board/5/messages/1319.html To ride the gravy train is to make easy money. I origanally thought it meant to make lots of money. I suppose easy money any and lots of money are similar. but thanks also to retroglide for providing an answer that concurs with the one I found.