Yes and no on the Master System/GameGear. There was an adaptor, the MasterGear Converter, that let you play Master System games on the GameGear. However, the GameGear had a higher color pallet, which means that the toolkit Sega is building for these games is probably based on a GameGear system.
Master System is just the "stronger" brand, I guess.
It's important to point out that while I do not encourage emulation, many emulators are capable of playing both. However, since every VC game is bundled with the customized emulator the issue is somewhat moot.