Gajeel is the 3rd strongest wizard in Fairy Tail. Gajeel is an Iron Dragon Slayer taught by the Dragon Metalicana. Gajeel is a strong headed tough guy who comes through in strength and faithfulness to Fairy Tail and his guildmates. Gajeel before becoming a part of Fairy Tail was apart of Phantom Lord who became an enemy to Fairy Tail in the Phantom Lord Arc. Phantom Lord was hired to attack Fairy Tail and abduct Lucy Heartfilia. Gajeel destroyed the guild and attacked Team Shadow Gear showing his hatred to Fairy Tail. Later in the Arc. Natsu and Gajeel fight it out of course Natsu won and destroyed Phantom's flying guild making the guild disband. Time after that Master Makarov found Gajeel and gave him a place in the Fairy Tail Guild. Gajeel not being a very liked person in Fairy Tail for his past choices was targeted by Team Shadow Gear beating him up for what he did to them but then Laxus comes and begins to beat him up even more harshly. Levy standing by and Jet and Droy trying to stop Laxus got Laxus mad and shot lightning towards Levy. Gajeel quickly went to Levy and stopped the lightning from hitting her proving that he doesn't want to harm anyone and be apart of the Guild.
Gajeel being taught by Metalicana the Dragon has a few tricks up his sleeve and yes he does eat Iron to boost up his powers even more. Gajeel uses many attacks such as, Iron Dragon Roar, Iron Dragon Iron Fist, Iron Dragon Sword, Iron Dragon Foot Blade, Iron Dragon Scales, Iron Dragon Secret Art Iron Spiral, Iron Shadow Dragon, Iron Shadow Dragon Roar, and Steel Shadow Dragon(Tartaros Arc.) Using these awesome and strong attacks is what makes Gajeel an Awesome Dragon Slayer.
In the Tartaros Arc. Gajeel is in a battle with one of the Nine Demon Gates of Tartaros named Torafusa. Torafusa has two huge blades on the side of his arms and is able to to turn into a huge piece of metal just like Gajeel. Gajeel turns into his Iron Shadow Dragon mode to deafeat Torafusa once in for all until Torafusa uses his curse magic and brings black water to the area where they are fighting. At first everyone thinks its water until they realize the water has poison causing Lucy,Juvia, and Natsu unable to move Gajeel is starting to run out of air and hope as well as he is getting weaker from the water as well. Levy sees Gajeel sinking in and gives Gajeel all of her remaining air making Gajeel wake up and see Levy being targeted by Torafusa. Gajeel springs back and saves Levy then tells Levy to make Air using her magic. Gajeel fighting with everything he's got gets trapped in the hand of the enemy and has a flashbackto his Phantom Lord days to when he would have been called from the magic council for doing bad stuff. Older Belno always tried to convince Gajeel to live for something better than doing bad stuff and to love life. Gajeel finally understood what she was saying ever since he joined Fairy Tail and met Levy. Elder Belno always tried to help Gajeel because he reminded her of her dead son. Gajeel looking back at that fought for what Older Belno always told him for people he cares about and to live his life for his friends. Gajeel absorbed the carbon dioxide in the water and became steel and defeated Torafusa. Gajeel is a very strong guy outside and inside his heart.