Well, I've got some news for you.
Most sites do not have .gov backlinks and most sites do quite well.
Unless you're sure that the .gov backlink provider that you are thinking of using will deliver editorial links, then you shouldn't even bother with getting .gov backlinks.
By editorial links, I'm talking about actual articles on government web pages that link back to your site.
These must be editorial pages, like real articles, not profile pages or sidebar links.
When it comes to link diversity, most sites do okay with different .com, .net, .org, or .edu backlinks.
Depending on your niche, I don't really see your site needing .gov backlinks.