Greyhawk expands in interesting ways on the Original D&D.
Most notably it introduces the Thief with all of that classes abilities. Also Paladins are added as a sub-class of Fighter.
Races get a bit more of a description and half-elves are introduced.
Next up are modifiers for Ability scores. For Strength - to hit and damage modifiers are now given and the percentile dice for those with 18 strength. People with Dexterity 14 get -1 AC but it’s just couched as the attacker getting -1 on their roll. And high Constitution grants you pluses on your hit dice.
We finally get different amounts of damage according to the weapon! Likewise, a table for monsters showing number of attacks and damage per attack.
There are a whole bunch of new spells, some for existing levels but also whole new spell levels. Some of them seem very unbalanced. There’s a Strength spell which grants a Fighter a big increase in their Strength ability for 8 hours! Also there are spells that multiply the effects of other spells - eg Permanence.
A whole bunch of well-know monsters are also introduced - I won’t list them all because every one of them is a classic so I’d be listing all of them - suffice to say Beholder is one of them. Basically they’re pretty much all high level monsters.
The list of extra treasures is extensive and contains a lot of dodgy ones - it seems every third item runs something like “this looks like the previous (good) item but when used the player … [is attacked][becomes weakened][dies immediately with no saving throw][something else nasty]”. And my favourite name goes to the “Spade of Colossal Excavation”!
One other odd, thing - in most cases, time is talked about in terms of turns but just occasionally there’s a mention of melee rounds - it seems things weren’t thoroughly bedded down to the new ‘alternative’ combat system.
Lastly I can only assume the supplement is called Greyhawk because it has a bunch of the things Gygax had been using in his Greyhawk campaign. Otherwise, there’s no actual information about the Greyhawk setting or dungeon apart from two or three throwaway references.
With the three volumes of rules for the original D&D published in 1974, supplements for the game started coming out in 1975. Looking at the supplement's name you might think this is some kind of campaign setting for D&D, after all Greyhawk is one of the most famous settings in the game and the first one of them to exist, before better known settings like Forgotten Realms or Dragonlance came along. If you think this is that you are wrong, though.
This is very much a supplement in the real sense of the word, it's just adding stuff to the original rules. Adding more monsters, more traps and more items above all. This also adds more classes and in this supplement you get the Thief and Paladin classes. Also added is the ever popular Half-Elf player race and the percentage number after your strength ability number if you had 18. This last one would eventually leave D&D with the third edition, but it did hit me in the nostalgic feels, as I started playing in the second edition with a barbarian with a strength score of 18 (57)... so something for us with some gray hair.
There is little reference to Greyhawk as a setting, really, with only a couple of off handed remarks as examples about Gygax's experience when running his game as well as a drawing of a stone face in Greyhawk castle, up to the wonderful standard of the illustrations in the original rules. Definitely worth taking a look at, but don't go in expecting some Castle Greyhawk lore.