Spikes: Skarmory, Omanyte/Omastar
Heal Bell: Lapras, Articuno, Dragonite, Mareep/Flaafy/Ampharos, Lickitung
Baton Pass: Togepi/Togetic, Beedrill, Spearow/Fearow, Rapidash, Dodrio, Hypno, Zapdos, Natu/Xatu
Thunder Wave: Rattata/Raticate, Gyarados, Togepi/Togetic, Natu/Xatu, Aipom, Murkrow, Misdreavus, Girafarig, Dunsparce, Snubbull, Granbull, Qwilfish, Remoraid/Octillery, Stantler, Miltank, Raikou, Tyranitar, Lugia, Ho-oh
Sing: Cubone/Marowak, Kangaskhan, Marill/Azumarill, Pichu/Pikachu/Raichu
Swords Dance: Rhyhorn/Rhydon, Totodile/Croconaw/Feraligatr, Ledyba/Ledian, Hoppip/Skiploom/Jumpluff, Sunkern/Sunflora, Gligar, Qwilfish, Heracross, Sneasel, Teddiursa/Ursaring, Celebi
Soft-Boiled: Cleffa/Clefairy/Clefable, Togepi/Togetic
Earthquake: Venusaur, Mankey/Primeape, Tyrogue/Hitmonlee/Hitmonchan/Hitmontop, Chansey/Blissey, Pinsir, Gyarados, Dragonite, Mewtwo, Pineco/Forretress, Dunsparce, Snubbull/Granbull, Mantine
Rock Slide: Omanyte/Omastar, Mantine, Tyrogue/Hitmonlee/Hitmonchan/Hitmontop, Lickitung, Pinsir, Kabuto/Kabutops, Aerodactyl, Typhlosion, Pineco/Forretress, Gligar, Shuckle, Heracross, Ursaring, Skarmory, Donphan, Miltank
Selfdestruct: Qwilfish, Slugma/Magcargo
Explosion: Corsola
These are all the important ADV>GSC tradeback moves I could think of. Also, another gain is move compatibilities. Any Gen 2 exclusive egg moves or event moves prevented trading back to Gen 1 to access the TMs there, but now you could trade them to Gen 3 to learn those moves and then trade them back to Gen 2. For example, you could now have a legal Blissey set with Seismic Toss/Present/Thunder Wave/Soft-Boiled.