You could add h1 salary info (which is publicly available) to improve accuracy on the base compensation portion.
Glassdoor lets users filter reviews based on company location, which appears to take care of that problem.
As much as GD gets under my skin, it has actually been useful to me. I looked up a firm and saw that many reviewers thought the CEO was nutz (along with many astroturfed reviews). After a 2 hour phone interview (5-7pm on a Friday) I left a similar review. Gotta pass it along.
One thing that GD could do is launch into the #MeToo movement. Instead of excel files, having GD as a clearinghouse would be useful. The glaring libel and slander issues aside, it would be an exceedingly brave step, an unlikely one for GD, but a very useful one.