The Gravity suit is a suit that is generally obtained after you acquire the Varia suit. It allows Samus greater mobility in the water and also allows Samus to survive in the lesser heated pools of lava. You still take damage, however if you fall into the super heated acid that is generally found in Lower Norfair.
After acquisition, Samus turns a purple colour.
 Super Metroid
The Gravity suit is a very important upgrade as it allows you to explore Mardia without restriction, allows you to take more damage and also allows you to thrive in the lesser heated magma, which is integral to access Ridley's Lair in Lower Norfair as the room before his elevator is filled with this magma.
Beware, however that the Gravity suit does not protect you from the Super heated pools of magma in Lower Norfair, or the acid found in Tourian.
 Metroid Zero Mission
The Gravity suit in this game works exactly the same way as in Super Metroid. The only difference is that in this game, The Gravity suit is one the "unknown items" with this item being located in Ridley's Lair.
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