The green kind that you are rewarded with for doing a job. Especially when it is a job (meat and potatoes) that nourishes your creativity, sense of emotional security, and satisfaction in a job well done. The gravy pays the bills, but it is the meat and potatoes make it worthwhile...when it is the right job. When a person is working at a job that does not provide them with the meat and potatoes, they are struggling with a diet of gravy and no substance.
Every coin has two sides. For some their art/creativity is their meat and potatoes that proffers them a way to earn gravy while doing what they truly love. For others, the gravy is what allows them to express their creativity in areas that cost gravy instead of supplying it.
I'll say that the meat and potatoes of life is doing what you must do, your profession, whether you enjoy it or not. It's the necessities of life.... Gravy is doing or having whatever gives you joy. Hopefully they can be found on the same plate and eaten together.
Since I retired, my life has been pretty much all gravy.
Nancy, I think that is truer to than the original quote for most of us most of us. The original quote is from the view of a successful artist. For her, work was her joy and satisfaction. That she got paid to do it was a happy addition. I think most successful artists would agree with her. Struggling artists strive to reach that goal. I think most of us relate more to Jon's and your view which is the reverse.