I can confirm that after 235 successful melee hits during the GCD (activated using find beasts) I did not get a single Nightfall proc. Same should be true for many other weapon procs and weapon imbues such as Coldrage Daggers and Frost Oils as they are on the GCD. This explains why Nightfall has a lower proc rate in raids (where we use abilities that activate the GCD) compared to target dummy tests. Here is a list of affected weapons gatherd on the fightclub discord by tilkku.
GCD IID Item name SID Spell name
1500 1318 Night Reaver 13480 Shadow Bolt
1500 810 Hammer of the Northern Wind 13439 Frostbolt
1500 880 Staff of Horrors 8552 Curse of Weakness
1500 937 Black Duskwood Staff 18138 Shadow Bolt
1500 869 Dazzling Longsword 13752 Faerie Fire
1500 997 Fire Sword of Crippling 89 Cripple
1500 1481 Grimclaw 13440 Shadow Bolt
1500 1482 Shadowfang 13440 Shadow Bolt
1500 1982 Nightblade 18211 Shadow Bolt
1500 3822 Runic Darkblade 16409 Shadow Bolt
1500 2256 Skeletal Club 13440 Shadow Bolt
1500 2299 Burning War Axe 18199 Fireball
1500 3687 Deprecated Unholy Avenger 13438 Fireball
1500 1387 Ghoulfang 16409 Shadow Bolt
1500 2263 Phytoblade 14119 Lightning Bolt
1500 2205 Duskbringer 18217 Shadow Bolt
1500 3194 Black Malice 18205 Shadow Bolt
1500 2163 Shadowblade 18138 Shadow Bolt
1500 2912 Claw of the Shadowmancer 16409 Shadow Bolt
1500 2915 Taran Icebreaker 16415 Fireball
1500 3854 Frost Tiger Blade 13439 Frostbolt
1500 6331 Howling Blade 13490 Howling Blade
1500 2942 Iron Knuckles 13491 Pummel
1500 4985 Test Proc Wand 2607 Shock
1500 6622 Sword of Zeal 8191 Zeal
1500 6831 Black Menace 13440 Shadow Bolt
1500 9446 Electrocutioner Leg 13482 Lightning Bolt
1500 8006 The Ziggler 13482 Lightning Bolt
1500 9386 Excavator’s Brand 13438 Fireball
1500 9651 Gryphon Rider’s Stormhammer 18081 Lightning Bolt
1500 10797 Firebreather 16413 Fireball
1500 754 Shortsword of Vengeance 13519 Holy Smite
1500 10625 Stealthblade 12685 Fade
1500 11902 Linken’s Sword of Mastery 18089 Lightning Bolt
1500 10623 Winter’s Bite 13439 Frostbolt
1500 9639 The Hand of Antu’sul 13532 Thunder Clap
1500 11121 Darkwater Talwar 16408 Shadow Bolt
1500 13051 Witchfury 18214 Shadow Bolt
1500 811 Axe of the Deep Woods 18104 Wrath
1500 13032 Sword of Corruption 17510 Corruption
1500 12992 Searing Blade 16413 Fireball
1500 10761 Coldrage Dagger 13439 Frostbolt
1500 9486 Supercharger Battle Axe 13527 Lightning Bolt
1500 8225 Tainted Pierce 13530 Corruption
1500 10803 Blade of the Wretched 18088 Corruption
1500 12621 Demonfork 16603 Demonfork
1500 13361 Skullforge Reaver 17484 Skullforge Brand
1500 13348 Demonshear 17483 Shadow Bolt
1500 12792 Volcanic Hammer 18082 Fireball
1500 12250 Midnight Axe 13440 Shadow Bolt
1500 12243 Smoldering Claw 15662 Fireball
1500 13408 Soul Breaker 17506 Soul Breaker
1500 11817 Lord General’s Sword 15602 Lord General’s Sword
1500 12974 The Black Knight 14106 Shadow Bolt
1500 13053 Doombringer 18211 Shadow Bolt
1500 14576 Ebon Hilt of Marduk 18656 Corruption
1500 14145 Cursed Felblade 18381 Cripple
1500 15418 Shimmering Platinum Warhammer 19874 Lightning Bolt
1500 17071 Gutgore Ripper 21151 Gutgore Ripper
1500 17752 Satyr’s Lash 18205 Shadow Bolt
1500 17068 Deathbringer 18138 Shadow Bolt
1500 870 Fiery War Axe 18796 Fireball
1500 17182 Sulfuras, Hand of Ragnaros 21162 Fireball
1500 19910 Arlokk’s Grasp 18205 Shadow Bolt
1500 19169 Nightfall 23605 Spell Vulnerability
1500 19170 Ebon Hand 18211 Shadow Bolt
1500 17193 Sulfuron Hammer 21159 Fireball
1500 18881 TEST Ragnaros Hammer 21162 Fireball
1500 19158 TEST Sulfuras, Hand of Ragnaros 21162 Fireball
1500 19100 Electrified Dagger 23592 Lightning Bolt