Gamma Metroids are significantly larger and more dangerous than Alphas. They possess a lightning attack and are much more durable. Their attacks can deal substantial damage.
Strategy: Gamma Metroids are best dealt with using a combination of charged shots and missiles. Aim for their exposed underbelly when they attack. Dodge their lightning bolts by jumping or crouching. It will take a good number of missiles to bring them down, so ensure you have a healthy supply. After defeating a Gamma Metroid, the lava will recede even further, opening up new sections of the ruins.