Gravy is a sauce made from the juices of meat or vegetables and enhances the flavour of a meal. In poorer times, to have gravy on your meal is a sign that you have sufficient meat and vegetables to make such a nice meal. Hence the phrase "on the gravy train
" meaning that ones life was well supplied with good things, usually money.
"It's all gravy" therefore means that there is an abundance of good things in the given circumstance.
It should not be taken to mean that there is no problem or that a situation is liked, specifically. Those things may follow but are not necessarily connected.
A: We just secured a lucrative contract with the supplier with an open ended term.
B: It's all gravy from here on, boys.