The rest is (just) gravy sentence
The rest of something is very easy to complete.
I think that the last question is pretty difficult. The rest is just gravy.
Crossing the desert is the hardest part of the journey. The rest is gravy.
Any additional money received is just an unexpected or unrequired bonus.
You only need to pay for the plane ticket, the rest is just gravy.
We only need to sell half of the product at this price. The rest is gravy.
The origin of this phrase is not clear.
Meaning absolutely certain or without a doubt
I am as sure as God made little green apples that Peter's the one for you, you two make a beautiful couple.