Actually, Gannon's hella-underrated. His career stats are startlingly good, considering he flew under the radar for most of his career. And he did have that awesome MVP season in Oakland in...what was it, '02?
In any case, if I were making a top 20 list, he'd probably be on it.
But yeah, not ahead of Manning.
....
Also, Otto Graham isn't on the list. If you have older guys like Unitas and Baugh, Otto should be ahead of them both based on accomplishments and numbers. And as for Unitas at #1, anyone who tells you that is either over 60 or deluded. He would compete with some of today's QB's, but neither his accomplishments nor his stats nor his dominance of the era compares with MANY others on that list.