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For RBY to GSC, definitely the better RBY TMs- Body Slam, Submission, Reflect (which a number of atypical Pokemon could learn in RBY), Rock Slide, Substitute, easier access to staples with Missingno. You could score some Pokemon from the Safari Zone or Seaform much earlier in the game at an acceptable level range to when you beat Morty.
For GSC to RBY, the only things I can think of off hand are the elemental punches as TMS (though a number of Pokemon got their Special Attack boosted in GSC and cannot use them nearly as well in RBY), and that breeding and certain GSC encounters net you a number of mostly non-evolving Pokemon for the Pika Cup rules (15-20), Arbok, Scyther, Magmar, Pinsir, Tauros, etc. Sadly the introduction of babies actually curbed just how much you could do this. I guess Headbutt could be useful as a universal flincher (does it flinch in RBY?), but you could be cloning the Body Slam TM. Some other earlier-access Pokemon I'm sure I'm forgetting.
