Back when the original folio edition of the World of Greyhawk was released, there was little discussion of religion in Oerth. In fact, despite several of the leaders of various areas being defined as Clerics, there is no mention of particular deities or religions at any point (Iuz excepted, but he's not called a god or even a demon in the folio's Gazetteer, as well as Zagig, similarly not termed a god or demigod at this point in time). This leaves me wide open as to defining the religions of Oerth. I want to include St. Cuthbert of the Cudgel and Pholtus of the Blinding Light, as these two are known to have been the original Lawful gods of Gygax's campaign. There are a few others implied, for which I will dip into the boxed set's descriptions for ideas, but will not feel bound to them.
First, I want to note that there's a significant difference in reality between monotheist and polytheist religions. These fundamental differences result in very different ways of approaching the world. To work with this idea, I want to limit some of the ways that each religion is portrayed in game terms. Only monotheist religions will have some of the abilities we usually think of as "clerical", such as undead turning or miraculous prayer. Polytheist religions will more closely resemble traditional gaming magic. So, the advantages True Faith, Blessed, and Divine Favor (from GURPS Powers: Divine Favor) will be the basis of the monotheist clerics' magical abilities, while the abilities of polytheist "clerics" will be simply the basic magic system. In the case of Druids, the reorganization of the basic magic system as the "Tree Magic" system from GURPS Thaumatology will be used. OK, that gets most of the game-specific material out of the way, so let's discuss the religious landscape of the Flanaess.
The monotheist religions are those of St. Cuthbert and Pholtus, certainly, but I am also inclined to present the religion of the far Northwest (Ekbir, Zeif, and so on) as monotheistic, but I'll come back to that. My general impression is that Pholtus is the religion of most of the Flanaess. Edit to add: Pholtus is mainly concentrated around the Great Kingdom, the Pale, Nyrond, Urnst, and so forth. End edit. St. Cuthbert, on the other hand, is the god followed by those in the central Flanaess, such as Greyhawk and Verbobonc, Furyondy and Veluna. I'd say that his worship extends into the area of Keoland and surrounding areas. My impression is that the religion of Pholtus is more inflexible and intolerant than St. Cuthbert's, but the two religions are also specifically active rivals for worshipers. St. Cuthbert is not as intolerant of the polytheists around, but Pholtus is actively trying to eliminate them and any other religion as heretics.
The Baklunish nations in the far Northwest, on the other hand, have been portrayed as polytheist since the boxed set. However, their implied culture is based on one of the most monotheist cultures in real-world history, the Islamic Arabs. The boxed set lists several gods as being Baklunish in origin: Istus (the Lady of Fate), Geshtai (goddess of lakes, rivers, and wells), Xan Yae (goddess of twilight, shadows, and such), and Zuoken (god of physical and mental mastery). I want to incorporate those into the world, so I will imagine that Istus is the True God of the Bakluni. The others will be Archangels fulfilling the Will of the Lady of Fate. There will be other Archangels, but I won't worry about those until I need them. The Bakluni will believe that other monotheist gods are merely different ways of envisioning the True God, Istus, but that polytheists are deluded demon-worshipers. Istus's worship extends into Ket and the areas of the Tiger and Wolf Nomads.
There are three major polytheist religions: Oeridian, Flan, and Suloise. The Oeridians are the religion that Pholtus came from, and they perceive Him as one god among many. The monotheist Pholtus-followers are particularly aggressive about their "backward, demon-worshiping" cousins. Their main gods include Zilchus, the god of power and influence, and Procan, the god of the oceans. There are several other gods, dedicated to aspects of nature in the main, but also notably Heironeous, the god of justice and chivalry, Delleb, the god of the intellect, and Hextor, the evil god of war and discord. Oeridian "clerics" are not particularly distinguishable from magic users, and learn spells as normal. Oeridian religion is scattered in the area of the Great Kingdom (though oppressed by Pholtus monotheism), the Shield Lands and the Bandit Kingdoms, and the areas of Keoland and the surrounding areas.
The Suloise barbarians of the Northeastern peninsula are listed in the boxed set as having three major gods: Kord, god of sports and brawling, Lendor, god of time and tedium, and Wee Jas, goddess of magic and death. Kord is probably their most important god. Their clerics are also very much like traditional magic users. Other areas where Suloise gods hold sway include Hepmonaland and the Amedio Jungle, Keoland and the surrounding areas (mixing with the Oeridian temples in a harmonious way), and the Scarlet Brotherhood's peninsula. In addition, they can be found pretty well represented in the central Flanaess alongside St. Cuthbert's chapels.
Flan religion is almost certainly the Druidic religion of Oerth. The boxed set lists four gods as the major ones of the Flan pantheon. They are Beory, the Oerth Mother, Nerull, god of death, Pelor, god of the sun, and Rao, god of peace and serenity. Iuz is part of this pantheon, and is the incarnate god of oppression, deceit, and pain. Their clerics will use the Tree Magic version of the basic magic system. I may rename the 18 oghams of that system with symbols from the "Glossary of Portentous Runes and Glyphs" of the Gazetteer, but that's pretty low priority. These Druids can be found throughout the Flanaess, but are especially common in Tenh and the Barrens.
So, if polytheist clerics are "merely" magic users by another name, why do they have temples and shrines? Like wizard towers, shrines and temples give a place for priests to live and receive visitors and petitioners. In addition, they give a place for common worship to occur. This has few game system effects, being mainly the way that communities bond and improve their communal order. However, using the GURPS magic system, they can also serve as the location of Ceremonial Magic casting for major spells like Bless Plants.
There are other gods listed in the boxed set as "Common", such as Boccob, Incabulos, Cyndor, Bleredd, Ehlonna, Joramy, Lirr, Myrhiss, Olidammara, Ralishaz, Tritherion, and Zagyg. I'm going to figure that these are, like Zagyg/Zagig, powerful entities similar to the major demons or the powerful modrons or else local gods (so that Ehlonna, for instance, is a goddess whose worship is found in the Gnarley Forest, the Welkwood, and the Suss Forest only). Generally speaking, in fact, these gods are found in specific areas (Iuz in his own land, Zagig mainly in Greyhawk, and so on).