Is "God knows I'm an atheist" a tautology/contradiction/both/neither?

It is a statement that is not redundant so it is not tautology.

It is in direct contradiction if the statement is uttered by one whom is an Atheist and understands Atheism.

It is more likely to be spoken by one whom misunderstands the word Atheists and is still searching for their path in life. Kirk Cameron considered himself an Atheist, yet has no understanding what the word itself means . . . He mentioned he was "looking" for something and went to some various churches. God changed him from and Atheist to a Christian. The fact that Kirk had Looked for something at various theistic religious institutions, means he most definitely believed in a power/presence that is outside himself and all powerful . . . The very definition of a God . . . It is more likely he was a theists looking for the particular God he would worship.

Sounds like a contradiction in terms to me. On the other hand, "Thank God I'm an atheist"  is an attempt at humour.

Of course, if there were really a God, he wouldn't penalise me by putting a wobbly red line under "humour" and "penalise"  Everybody knows that God is an Englishman, not an American, so why wouldn't he spell like one?

That may not be true of Jesus because Americans (and only Americans) are fond of exclaiming Jesus H. Christ!  when they become exasperated. Hmm, that might create some problems for the Trinity concept. Ah well, back to the drawing board...

In a way, it's an oxymoron.

This statement is in direct conflict with itself; by allowing for there to be a God to know of one's atheism, the speaker could not truly be atheist. Saying comparatively 'God knows I don't believe in Him', is not atheism, it's only a shade of personal denial. Atheism is declaring plainly there are no gods at all, regardless of belief. 

True, but as Dark suggest, probably not accurate.