Gmane doesn't mirror google groups -- it mirrors public mailing lists and originally was made to reformat them for news readers[1]; some mailing lists use it intentionally as their public archives (emacs org-mode for one) so you might be missing out if you blacklist it. I don't have any affiliation with them, but they don't seem too bad.
Gmane was not around first. Google Groups started almost a year before the gmane.org domain was even registered, and it was built upon the purchase of the DejaNews archives, which itself began in the mid-90s.
Deja News was a web interface for Usenet, not a newsgroup interface for mailing lists. I don't remember when google groups switched to a web/mailing list setup, but I think it was later than 2002 (when Gmane started).
[1] http://gmane.org/about.php