They have a lot of good info, and are a cold analytical unit like engineers. Being responsible to both the White House and Congress, all political tints, they can't get far out of line. Of course they can look only at what they asked to look at, so the right questions must be asked. Ask your congresscritter and he might ask the GAO.
I can't quote you chapter and verse, but I have seen many GAO reports that firmly support a liberal slant.
They may be cold analytical engineers, but I guess the questions that produce the reports I refer to, are coldy calculated as well.