Uh, no. Do not need heading input to a G5, mostly... especially if the G5 is a backup.
What you really care about (almost all the time) is your course. Heading is used to help you achieve that course, but heading is subject to magnetic errors in the airplane, and winds can add an additional difference between heading and course. If there's no heading source, the G5 will display GPS course -- which is what you really want, and you don't have to keep trying different headings to get your desired course.
With a fast turn rate, GPS course may lag more than a heading source, but that's really a so what. And if you're on vectors, ATC will give headings but unless there is a significant crosswind, the difference between heading and course will not be significant. And ATC will adjust your assigned heading as required if you're flying course. Yes, there are possible legalities here, but operationally, it's like a DG that has drifted significantly.
Adding a GMU11 can be a real pain, especially when you consider running all the wires, etc. If the G5 is your only displayed heading source, yeh, makes sense. Otherwise...