A GI shower is a code red used to enforce hygiene in the unit. The marine (or airman, or sailor, or soldier) who has failed to bathe on a regular basis is trained or motivated--in the Pavlovian and Darwinian senses--to shower of his own initiative, lest his comrades decide to shower him on their own. A GI Shower involves scrub brushes, steel wool, brillo pads, and anything else remotely abrasive, like Bon Ami scouring powder. Cold water is optional; it can enhance the terror and soften the skin, which may or may not be a good thing--YMMV.
The offending party is awakened in the middle of the night by his unit and dragged to the bathroom, where he is stripped naked. Water may or may not be applied, and then the unit takes turns "cleaning" the comrade with the aforementioned implements. He is left to tend to his wounds, and as with all code reds, it is implicit that he will not discuss the origin of the injuries, or perhaps simply answer that "This marine has taken a GI shower." Implicating anyone else for this will probably garner the GI a sock party or similar retribution.