PC + Clone Hero / Phase Shift: Map gamepad buttons to frets (e.g., D-Pad + Face buttons). Playable but not fun — no strum, no tilt, no velocity.
Third-Party Adapters (Rare):MIDI Pro-Adapter (RB3) lets you use real MIDI guitar/keys. No gamepad→instrument adapter exists.
Button Codes (Cheats): Some games have "Auto-Play" / "No Fail" cheats entered via instrument — not gamepad.
Best Solution
Buy used instruments: Guitar Hero 5 / Warriors of Rock guitar (universal, 5-button, works RB1-4). RB2/3 Drum Kit (velocity pads). USB Mic (any Rock Band / Guitar Hero / SingStar).
RB4 (PS4/XB1): Legacy adapter needed for old wireless instruments. Wired USB works native.
Rock Band is a peripheral game. The controller is the instrument.