Question
I have a 7 foot high cinder block retaining wall that was installed in front of an older cement block retaining wall that started to bow out.  There is a 5 inch space between the 2 walls.  Should this space be filled in with gravel? cement? or left alone?

Answer
Was the new wall installed with any steel reinforcing in the cores and then grouted with concrete? Was the new wall built with a substantial footing going back about 30" or more on the side where all the dirt is? Since you say there is an old wall behind it, I suspect not.
If not, the new wall will likely fail as the old wall pushes into it because it was not engineered or constructed properly. Without steel and a footing tied in with steel the wall is just like a bog board standing straight up. It has nothing to hold it there but its own weight.

But to answer your question, you don't need to to anything with the space between except to close it off from critters in whatever way works for you. However I suppose that adding concrete might make the hole apparatus stronger.

Good Luck