Yes, it probably means that they're a two income family making a combination of GS salaries that adds up to 28. Never heard the phrase before, and we are such a couple (okay, a GS 29)

I have heard the income of a Federal employee couple referred to as a GS 28? Is this some kind of addition of 2 GS scales? Like a 13 and 15

Does this have some kind of symbolic importance?