Medi-Gel in Mass Effect 2 is an consumable item for health regeneration, it is basically the only consumable item in the game. It is used for Commander Shepard and their Companions. But it's also used for NPCs in some cutscenes.
Medi-Gel is used by the new automatic self-healing system controlled by the armor suit device. It can also be used in combination of the required upgrades, to restore full health and shields for the required characters, Companions or Commander Shepard, it can also be used to revive squadmates.
Allies can be revived by administrating a Medi-Gel unit on the Unity Power. It will revive the downed mates, but given enough research Medi-Gel will be also able to restore health and shields of Commander Shepard or Companions. When Medi-Gel is used to revive, it will cost one Medi-Gel unit per use, if both squadmates are down, one revive use will revive both of them still consuming only one Medi-Gel.
The Unity heal effect can be interrupted if Shepard gets stunned or staggered during their Unity casting animation, but if it's not interrupted during that period, once the Unity heal effect successfully begins it can't be interrupted anymore.
How to get
Medi-Gel can be obtained by opening aid containers or stations, which are found around every map in every area, Medi-Gel is quite abundant in Mass Effect 2 and it shouldn't be hard to find, even during Boss fights.
It can be stacked in the inventory up to 3 Medi-Gel units. Research and Upgrades may allow Commander Shepard to expand the Medi-Gel units capacity up to a maximum of 9 Medi-Gel units capacity in Shepard's inventory.
Notes & Tips
By opening a Medi-Gel container when Commander Shepard is carrying the maximum number of Medi-Gel units, Commander Shepard will obtain 100 credits instead. Medi-Gel is not refilled between missions and can only be obtained during Missions.