Furthermore, I did a check of headlines at the time the Plame biz was in the news. The focus at that point, via headlines, was predominantly upon "Africa harboring terrorists" -- not necessarily Africa being a harborer or conduit for uranium. The latter focus had already begun earlier, and I suspected an attention redirect away from countries more obviously in suspicion of harboring/conduiting Uranium - Syria, Iran, etc.
The whole of it struck me as a DC, foggybottom, redirect of national attention via "rumor".
I can tell you how he knew about the memo and was able to ask Wilson about it. About ten days before Gannon interviewed Wilson the WSJ had published an article describing...the memo. I therefore deduce Gannon read the paper.