Is metroids arm his arm or is his arm just attached to his gun?

It's just a cannon placed over her arm.

In the Metroid games, Samus (the woman controlled by the player) uses a special suit given to her by an ancient race. The suit materializes over her, including the cannon. She holds her hand in different positions to control what kind of beam/missle she fires. If you play Metroid Prime, you can actually use the xray visor to see her operating it.
It's just a cannon placed over her arm.

In the Metroid games, Samus (the woman controlled by the player) uses a special suit given to her by an ancient race. The suit materializes over her, including the cannon. She holds her hand in different positions to control what kind of beam/missle she fires. If you play Metroid Prime, you can actually use the xray visor to see her operating it.