I'm writing a roster editor and it would be very helpful if there's something like Magic the Gathering's Gatherer but for Warhammer 40,000 models.
I sincerely doubt there is, at least not officially. I've been playing Games Workshop games for the past 15 years, and their protection of intellectual property has gotten pretty severe in the past couple of years. Just do a Google search for "Games Workshop sues" and see all the fun that auto-complete brings up.
From their Legal Page, Bandwidth Theft and Mirroring Resources section:
And from the Online Auctions:
Now, I am not a lawyer, but between Privateer Press, Chapterhouse Studios, the TalkFantasyFootball forums, etc., etc., I would be very careful in what you do.