This is a neat resource. It lists off articles in Dragon magazine by author or topic.
Unfortunately (for the purposes of this search) Gary wrote a lot in Dragon. The list below has article names in quotes, followed by issue (page) and general topic. I marked a couple which were Greyhawk related, but there are probably others. The format isn’t great, due to copy-paste. Hope it helps, though.
"50 Years of D&D Gaming" An4(8) D&D2
"AD&D's Magic System: How and Why It Works" 33(10) D&D1
"Adding Depth To the Flanaess - Appendix" 52(18) D&D1"Ain't It the Pits?" 294(18) --
"All I Need To Know I Learned From D&D" 287(26) --
"All I Need To Know I Learned From D&D" 288(24) --
"Ancient & Medieval Standard Military Symbols" SR7(16) --
"Astral Plane, The" 67(27) D&D1
"Barrage Balloon? The End of Murlynd's Mayhem" 317(12) --
"Bazaar of the Bizarre: Bag of Wind" 27(46) D&D1
"Best Shots: The Folly of Predictability" 306(16) --
"Big, Bad Barbarian, The" 63(8) D&D1
"Bob the Warrior" 276(10) --
"Books Are Books and Games Are Games..." 31(28) D&D1
"Calling All Fanboys" 267(16) --
"Cantrips For the Aspiring Illusionist" 61(6) D&D1
"Cantrips: Minor Magics for Would-Be Wizards" 59(6) D&D1
"Changing Play, The" 275(8) --
"Character Classes To Consider" 65(9) D&D1
"Chivalrous Cavalier, The" 72(6) D&D1
"Clerics Live By Other Rules" 92(6) D&D1
"Conan!" 36(10) D&D1
"Conventions: The Manufacturer's View" 28(19) --
"Cut To the Quick" 320(16) --
"D&D, AD&D, and Gaming" 26(28) --
"D&D Ground and Spell Area Scale" 15(13) OD&D
"D&D Is Only As Good As the DM" SR7(22) OD&D
"D&D: The Next Generation" 273(22) --
"Deities & Demigods of the World of Greyhawk, The" 67(23) D&D1
"Deities & Demigods of the World of Greyhawk, The" 68(63) D&D1
"Deities & Demigods of the World of Greyhawk, The" 69(27) D&D1
"Deities & Demigods of the World of Greyhawk, The" 70(20) D&D1
"Deities & Demigods of the World of Greyhawk, The" 71(52) D&D1"Deities and Their Faithful" 97(8) D&D1
"Demi-Humans Get a Lift" 95(8) D&D1
"Developments From Stonefist To South Province" 57(13) D&D1"Devious DM, The" 307(16) --
"Direction Sense" 293(18) --
"Does Anyone Remember War of the Empires?" 3(4) --
"Dragonchess" 100(34) Dragonchess
"Dual To the Death" 319(14) --
"Dungeon Hospitality" 292(24) --
"Dungeon Master's Guide Developer Notes, The" 28(4) D&D1
"Dungeons & Dragons Magic System, The" SR7(3) OD&D
"Dungeons & Dragons: What It Is and Where It Is Going" 22(29) OD&D
"Elementary Ideas for Elemental Adventuring" 47(9) D&D1
"Events Of the Eastern and Southern Flanaess" 64(13) D&D1
"Everything You Never Knew About Spellbooks" 62(14) D&D1
"Evil: Law Vs. Chaos" 28(10) D&D1
"Fastest Guns That Never Lived, The" SR7(6) Boot Hill
"Fastest Guns That Never Lived, I-IV" 36(44) Boot Hill
"Featured Creatures" 63(5) D&D1
"Featured Creatures" 64(10) D&D1
"Featured Creatures" 65(25) D&D1
"Featured Creatures" 66(20) D&D1
"Featured Creatures" 67(10) D&D1
"Featured Creatures" 68(4) D&D1
"Featured Creatures" 69(36) D&D1
"Few More Words on Medieval Pole Arms, A" SR4(2) OD&D
"Final 4! (RPG Elements), The" 283(28) --
"Final What the Heck is a Roleplaying Game, The" 286(26) --
"First Orc Hero, The" 312(16) --
"Fly Me To the Moon" 301(16) --
"Founding Greyhawk" An2(96) --"From the Sorcerer's Scroll" 122(40) D&D1
"Future of the Game, The" 103(8) D&D2
"Games Fair '82: Everything Convention-Goers Could Ask" 63(59) --
"Gamma World: A New List of Treasures To Be Found" 19(8) Gamma World
"Gary Gygax on Dungeons & Dragons" 7(7) OD&D
"Getting Even Puts the Other Guy Ahead" 305(14) --
"Good Isn't Stupid, Paladins & Rangers..." 38(22) D&D1
"Goristro Revealed, The" 91(8) D&D1
"Greyhawk's World" 71(19) D&D1
"Greyhawk: The Shape Of the World" 37(10) D&D1"Hack & Slash" 270(22) --
"Half-Ogre: Smiting Him Hip and Thigh, The" 29(12) D&D1
"Hobbits and Thieves In Dungeon!" 1(25) Dungeon!
"Hold That Person!" 90(16) D&D1
"Hooray For the Dungeon Crawl!" 272(22) --
"How Green Was My Mutant" 5(16) Metamorphosis Alpha
"How To Train a Black Pudding" 289(28) --
"Incomplete Villain, The" 300(16) --
"Influence of J.R.R. Tolkien On the D&D and AD&D Games" 95(12) D&D1/BD&D
"Inner Planes, The" 73(10) D&D1
"Irresistible Force" 22(8) --
"Life Beyond 15th Level" 93(8) D&D1
"Loyal Readers" 67(67) D&D1
"Making Monsters Meaningful" 42(16) D&D1
"Maybe Even Dull Witted" 271(22) --
"Meaning of Law & Chaos in Dungeons & Dragons" SR6(3) OD&D
"Medieval Pole Arms" SR2(5) OD&D
"Melee In D&D, The" 24(17) OD&D/D&D1
"Missile Weapon In Classic Warfare, The" SR7(18) --
"More Cantrips: Mini-Spells for Apprentice Magic-Users" 60(16) D&D1
"More 'Meat' For Greyhawk" 55(17) D&D1"Munchkins, Anyone?" 269(22) --
"Nerds, Geeks, and Worse" 268(21) --
"New Denizens of Devildom" 75(8) D&D1
"New Druid Spells...Naturally!" 71(5) D&D1
"New Jobs For Demi-Humans" 96(8) D&D1
"New Magic-User Spells" 67(4) D&D1
"New Spells For Illusionists" 66(22) D&D1
"New Weapons From Another 'Great Mind'" 64(6) D&D1
"Nomenclature of Pole Arms, The" 22(50) OD&D
"Official Changes for Rangers" 94(8) D&D1
"Official Errata..., The" 35(4) D&D1
"One That Got Away, The" 290(22) --
"Outfitting the New Agent" 60(60) Top Secret
"Penny For Your Thoughts" 302(20) --
"Perfect Dungeon Master, The" 274(8) --
"Place It, And They Will Come" 296(18) --
"Planes" 8(4) OD&D
"Playing On the Other Planes of Existence" 32(12) D&D1
"Poker, Chess, and the AD&D System" 67(63) D&D1/BD&D
"Pressing Situation, A" 313(14) --
"Proper Place of Character Social Class In D&D, The" 25(12) D&D1
"Protection Circles and the Like..." 56(18) D&D1
"Putting Together a Party On the Spur of the Moment" 26(37) D&D1
"Rail Baron: A Short Course for Empire Builders" 21(16) Rail Baron
"Random Generation of Creatures From the Lower Planes" 23(15) D&D1
"Realms of Role Playing" 102(8) D&D1
"Remember the Mission!" 304(14) --
"Rewards of Roleplaying, The" 297(20) --
"Robbing From the Really Rich" 299(18) --
"Roman Military Organization" 24(12) --
"See Africa and Die!" 20(18) Source of the Nile
"Self-Destructing PC's" 314(16) --
"Sherlock Holmes Or Sheer Luck?" 295(20) --
"Sneak Preview: AD&D Dungeon Masters Guide" 22(33) D&D1
"Social Status and Birth Tables: New For AD&D Play" 70(11) D&D1
"Solo Dungeon Adventures" SR1(3) OD&D
"Sooner Or Later, Luck Favors the DM" 291(20) --
"Spear In Man-To-Man Combat", The" SR1(3) Chainmail
"Special Knowledge and a Bureau for Infiltrators" 61(60) Top Secret
"Split Class For Nimble Characters: The Thief-Acrobat" 69(20) D&D1
"Sturmgeschutz and Sorcery" SR5(3) OD&D
"Summarizing the Results" 284(24) --
"Tractics Changes" SR1(2) Tractics
"Tractics Weapon/Vehicle Gun Changes" SR4(4) Tractics
"UK Revisited: Games Fair '83" 75(76) --
"Unearthed Arcana Additions and Corrections" 103(48) D&D1
"Update From the Chief" 101(8) D&D1
"Varied Player Character and Non-Player Character
"Alignment in the Dungeons & Dragons Campaign" 9(5) OD&D
"View From the Telescope Wondering Which End Is Which" 11(5) --
"Warhorses and Barding" 74(4) D&D1
"What's Ahead for TSR?" 35(12) --
"What's In a Magic Item?" 311(18) --
"What's In a Name?" 318(12) --
"What the Heck Is a Roleplaying Game?" 279(20) --
"What the Heck Is a Roleplaying Game? Part 2" 280(20) --
"What the Heck Is a Roleplaying Game? Part 3" 281(15) --
"What the Heck Is a Roleplaying Game? Part 4" 282(26) --
