The Gamma stage is an extension of the previous Alpha Metroid's physical structure. Along with an increase in size, durability and strength, the Metroid can now generate electricity to unleash mid-range lighting blasts as both an offensive and defensive measure. Eventually, the organism will shed its entire body to become a Zeta Metroid; the Gamma Metroid can thus be viewed as a mobile pupa with a developing young adult within.
Whereas the previous form had small barb-like growths along its sides, the Gamma Metroid's have now grown into longer, insect-like limbs. Despite this, the creature never touches the ground and perpetually floats in midair. The pair of compound eyes have multiplied into two groups of three and the exoskeleton has spread and thickened, giving additional protection to its membrane and nucleus underneath it. The creature's forehead horn and tusks have become larger and well-defined; it is with these that the Metroid seemingly controls its electric attack.
The remake, Metroid: Samus Returns, makes a few changes to the Gamma Metroids' physiology. They are now capable of using their legs to move along the ground and to jump with them as well. The horn and tusks have been changed into fangs for the new leech-like mouth that has been added between them. Their six compound eyes are red instead of blue.
Like all Metroids in Samus Returns, some Gammas are classified as "evolved". These evolved forms are visually characterized by their distinctive yellow-green shells. Their electrical attacks are much stronger than the standard forms and have an orange coloration.
In Return of Samus, the Gamma Metroid is capable of launching small arcs of static electricity from its head. This lightning attack can deflect Missiles as well as deal damage to Samus. It will constantly try to ram her against her as well. Gamma Metroids can withstand ten direct missiles.
In Metroid: Samus Returns, Gamma Metroids have additional attacks, such as shooting a long-range beam of electricity or fire from their mouths, releasing a damaging pulse around them, and conducting electricity or fire along the entire floor on which they stand. They will also occasionally drop electric or flaming projectiles from their membranes as they fly around. Their melee attacks will consist of a swooping dive attack from the air and a lunging bite when on the ground.
The Gamma is much more durable in Samus Returns, now taking 20 missiles to kill, can use electricity/fire to shield its mouth and underside from harm, and its shell is no longer a hitbox and will cause missiles to harmlessly bounce off. However, it can now be damaged by the Ice Beam, and shooting a charged Ice Beam shot at the creature's underside while it is levitating will freeze its underbelly and cause the Gamma to fall to the ground. If there are tunnel or hatch structures in the room and the Gamma takes enough damage, it will quickly dash and flee into the hole and retreat to another nearby room. Some Gamma Metroids will attempt to hide from Samus forcing her to flush them out by finding their hiding spots.
Like the previous stage, it is implied that Gamma Metroids emerge from the Metroid Husk of their Alpha Metroid stage, though this is event is not shown as Samus finds a shed Alpha Metroid Husk shortly before she encounters her first Gamma Metroid. If Samus successfully parries its attack with a Melee Counter she will mount it allowing her to fire Beams and Missiles at its head in a manner reminiscent of an Overblast, though the Gamma will eventually shake her off.
"After molting from the lesser mutation, the Metroids will grow much larger, gaining the power to attack with lightning bolts."
Nintendo Power issue 31
"These creatures are much larger than the Alphas and they attack intruders with stinging lightning bolts. Hit them quickly."
- Gamma Metroid (p. 21)
- "This is where the fight truly begins. Gamma Metroids don’t pick up any speed from the smaller, weaker Alpha phase, but they pack a massive punch. In fact, expect to be defeated again and again and again, until the Gamma beats its attack patterns into your head. It has more moves than Metroid larvae and Alpha Metroids combined, and each of them feels like getting hit by a world-class boxer. Play it safe when you see one of these mean monsters, and focus on landing attacks on its core with the Ice Beam and missiles. Once you learn its patterns, you stand a chance. Before that, it’s a fight from the ropes."
- The mutated Hopping Metroids' physical structure bears a resemblance to the Gamma Metroids'. Both possess a highly developed exoskeleton with four insect-like legs and their membranes are located underneath their bodies. The Hopping Metroid is stated to be an advanced mutation of the Phazon Metroid, which may suggest a similar metamorphosis as well.
- The remake Metroid: Samus Returns further adds to the resemblance between the Gamma and Hopping Metroid strains with the addition of a jumping attack to the former.