Using Game Boy Advance as GameCube Controller — Not Possible

No. Fundamental incompatibilities:

• Protocols: GBA and GameCube use different communication standards.
• Inputs: GBA lacks analog sticks, C-stick, shoulder triggers, proper button layout.
• No official support: Nintendo never enabled this.

Alternatives

• Third-party adapters (phone/tablet → GameCube via Bluetooth/Wi-Fi)—unreliable.
• PC Emulation (Dolphin): Map GBA emulator to virtual GameCube controller.

Use a real GameCube controller or WaveBird for best experience.