Frank Gore misses a few games every year because he's an RB, and that's what RBs do. He doesn't have anything degenerative or serious, RBs just tend to get dinged because it's the nature of the position. Gore's missed 5 games over the last 4 years, which is comparable to guys like DeAngelo (3 missed games in 2 years), and Peterson (2 missed games in 3 years). Even if you count his rookie year (where he was buried on the depth chart), Frank Gore has missed fewer games in his 5-year career (7) than Rashard Mendenhall has in his 2 year career (12, plus another game lost to suspension).
The reason everyone is always playing the "injury prone" card on Gore is because he had major reconstructive surgeries on his knees (both of them, iirc) in college, and his draft stock plummeted as a result. Still, at this point, he's been running on those knees for 5 years without the slightest issue, so I think we've reached a point where it's safe to say that those surgeries went well.
I like Gore as well, but this isn't completely accurate IMO. First, there's no reason to discount him missing time in his rookie season- if anything, him being buried on the depth chart means it should have been less likely that he missed time, but it wasn't. Secondly, it's not necessarily the number of games missed why he's thought of as injury prone, it's the number of times he's been injured. AP missed 2 games in his career, which was due to one injury and it was 3 seasons ago. Gore has missed time in 3 consecutive seasons and 4 of 5. AP has also played in 3 post season games in the last two seasons. I don't think there is much of a question who has been more injury prone, and that is one of the reasons why he isn't considered in the same tier as guys like AP, CJ3, or MJD, who haven't had the same issues (yet).