Giant Gar: Gar were first mentioned in OD&D as companions for Nixies. And I tell you, the stats here make any encounter with Nixies a darn sight deadlier. Giant Gars can also swallow opponents whole on an attack roll of 20, with a 5% chance per round that the swallowed character will die.
Gargoyles: Gargoyles first appeared in OD&D. Statistically, there are a few minor changes, but nothing noteworthy. They're still super-aggressive, even more so than before - instead of a 75% chance, they now have a 90% chance to attack anything they meet. And now they like to torture their prey to death, so gargoyles aren't getting any more pleasant as the game progresses.
Kapaocinths get a mention. They were introduced in Supplement II, along with a whole host of other superfluous aquatic variations of already existing monsters. We learn that they dwell in caves in shallow waters.
Gas Spores: It's another 'gotcha' monster designed by Gary to trick his players. A gas spore is a mindless floating sphere that resembles a beholder. If struck, it explodes and deals 6d6 damage to anyone within 20 feet. That's the part I remember. What I never knew is that if one touches you, it injects you with tiny 'rhizomes' that grow throughout your body and kill you unless you get a cure disease within 24 hours. If you die, your body sprouts a lot of baby gas spores. Awesome - I love monsters that turn the PCs into more monsters.
Gas spores are very cool, but their effectiveness has waned over the years, because every player's first instict upon seeing a beholder now is to think that it's a gas spore. That's probably their best use now - to lull players into a false sense of security before you unleash the real beholders on them.
Gelatinous Cube: Gelatinous Cubes were first mentioned in OD&D, but they didn't get stats until Supplement I. They now get a greater chance to surprise opponents, and their 'anesthetizing' power now gets a duration. Cubes used to be immune to cold, but now if they fail a save vs. a cold attack they take 1d4 damage and are slowed. They also get explicit immunity to sleep and hold spells now. Oh, and the aquatic Gelatinous Cubes mentioned in Supplement II are not to be seen here.
Ghast: I'm kind of shocked that Ghasts are making their first appearance here. A ghast is like a ghoul, but tougher. It has more hit dice, and the same paralysing touch (that even works on elves). Their main ability is their stench, which causes anyone who fails a save vs. poison to attack at -2. They have the standard undead immunities, but cold iron weapons deal double damage to them. They are also said to be often used as slaves by demons, so I wonder if they have some sort of demonic pedigree. Demons are the only other monsters I can think of that are affected by cold iron.
Ghost: Ghosts first appeared in The Strategic Review #3. They are basically the same monster, with a few minor tweaks. Characters of over 9th level are no longer immune to their aging attack, though those of over 8th get a +2 to their saving throw. And whereas before 5th level clerics were immune, now it is 6th level clerics. They can also now be struck by silver weapons when semi-materialised, though they only deal half damage. And they can only be hit by spells from an ethereal caster.
Ghoul: Ghouls first appeared in OD&D, and they are so rad that all Gary had to change was their bite damage (from 1d4 to 1d6). The more I read, though, the more I become convinced that ghouls are not 'dead' in the way that other undead are. They feel more to me like humans warped and twisted by their corpse-feeding tendencies.
Lacedons, the aquatic ghouls mentioned in Supplement II, are also present.