Like many things, the true impact of the deities is how they get used - and in the RPG sphere, especially in official products.
So, strangely enough, the very first Greyhawk deity to appear... is the Elder Elemental God!
Closely followed by Lolth!
(Of course, there are almost no details about the Elder Elemental God, and then a lot of details about Lolth).
Most of the early adventures are very agnostic - barely a deity to be found. This changes in the Village of Hommlet, which introduces the concept of Elemental Evil and its chief herald... a priest of Lolth.
Oh, wait. (Much ink, real and electronic, has been spent explaining that away).
The folk of Hommlet often revere St Cuthbert or have the druidical "Old Faith".
Finally, in the Temple of Elemental Evil we got to meet a
real god. And that god was St Cuthbert. And Iuz. And, honestly, you can run a lot of Greyhawk with just those two. Force of good, force of evil. All set!
The other god to make an appearance in early adventures is the chained god, Tharizdun, in a particularly unsettling adventure. No connection to the Elder Elemental God at this point. But that would change...
(I'm ignoring Tamoachan at present since it's outside the heartlands of Greyhawk).
Cheers,
Merric