Gajeel Redfox is the Iron Dragon Slayer, having been trained by the Iron Dragon, Metalicana. Gajeel was a troubled soul before joining Fairy Tail but, since he's become a member, he's found his home and his strength is a great asset to the Guild.
Gajeel constantly strives to prove his strength and he will fight to the death, no matter how strong the opponent. An iron-clad will is what this Dragon Slayer has used to make him one of Fairy Tail's strongest members, and you can definitely count on him against most opponents.
10 Easily Beat: GOD SERENA
Serena was widely believed to be Fiore's strongest mage and held the rank as the number one Wizard Saint as proof of that ability. When God Serena defected to the Alvarez empire, there was a panic as they knew how much of a danger he would be. Serena is strong enough to beat the combined force of four of the wizard Saints all by himself, with his eight different types of dragon slayer magic. These reasons highlight why God Serena outclasses Gajeel both as a mage and a Dragon Slayer.
9 Can't Beat: LEVY MCGARDEN
These days, Levy and Gajeel are a loving couple who will undoubtedly go on to have children and live a happy life together. But the early parts of their relationship were very different; a time when Gajeel was a more cold and callous man. Gajeel, while a member of Phantom Lord, attacked Team Shadow Gear and easily decimated them before leaving them in a humiliating position. Even though this fight would never happen in this story, it's pretty clear that, if it did, Levy would stand no chance.
8 Easily Beat: MAKAROV DREYAR
Makarov Dreyar is a member of the Ten Wizard Saints; easily one of the continent's strongest mages and the 3rd, 6th and 8th Master of Fiore's strongest guild, Fairy Tail. Though his power has dwindled since his peak years, Makarov is still strong enough to be able to easily best most of the mages he comes across.
On top of his brute strength, Makarov is also an intelligent tactician, who's able to read and adapt to the flow of battle. Though Gajeel is a promising mage, it is too soon for him to be able to best his own guild master in combat.
7 Can't Beat: PANTHER LILY
Panther Lily is an Exceed and the current partner of Gajeel Redfox, the two are practically inseparable and share a bond akin to brothers. Additionally, Lily is a member of Fairy Tail having formerly been a commander in the army of the country of Edolas. Lilly has been shown to be able to competently handled himself in battle, more so than any other Exceed throughout the series. Panther Lily has been shown to be competitive in a fight against Gajeel but, since then, Gajeel has experienced exponential growth, which means that would no longer be the case.
6 Easily Beat: AUGUST
Known widely as the "Wizard King", August was a man who mastered the use of many different types of magic and was reputed as the strongest male amongst Zeref's Spriggan 12. He is also the son of Zeref Dragneel and Mavis Vermillion, meaning his magical bloodline is impeccable, being the child of two of history's greatest mages. August has fought in numerous battles and, if it he ever went up against Gajeel, he would make light work of him. August has shown himself to be strong enough to go head to head against Gildarts, a man considered Fairy Tail's strongest mage, a man Gajeel could not last too long against.
5 Can't Beat: ELFMAN STRAUSS
Elfman is a longtime member of Fairy Tail. He’s a hardworking mage who does the best he can for his guild and family, whenever he can. Elfman, though not the strongest fighter in the guild, will always lend a hand when the guild finds itself against enemies. He wields takeover magic when its time to do combat but, unfortunately, none of his spells would be able to take out Gajeelin a one on one duel. Yes, Elfman's magic grants him massive power boost but it's arguable that Gajeel's dragon slayer magic is far more potent.
4 Easily Beat: METALICANA
The strength of dragons must not be underplayed in the world of Fairy Tail. When a dragon comes around, calamity often follows. As seen when the dragons returned, they have enough power to easily defeat some of the world's strongest mages. Made up of iron, Metalicana is easily one of the sturdiest dragons around. Even though Gajeel would be given a boost based on the fact that he is indeed a dragon slayer, that by itself would not make him capable of defeating a fully grown and battle sturdy Metalicana, not at his current power level.
3 Can't Beat: LUCY HEARTFILIA
Over Fairy Tail's serialization, we've seen Lucy Heartfilia grow as a combatant. A once timid and meek Lucy is now capable of fighting bravely until she has no more to give. This attitude has seen her eclipse many of her rivals as she chases down her dreams. Her Celestial Spirit magic grants her much variety, with each of her spirits boasting unique fighting styles and strengths. Lucy's spirit and combat strength mean that she would be able to give Gajeel some trouble but, ultimately, she would eventually fall to a stronger magician.
2 Easily Beat: ACNOLOGIA
Acnologia stands as the strongest Dragon Slayer in the series, he managed to defeat God Serena, a man that Gajeel would surely lose against, in one hit. This is a man so strong that the dragons themselves had to devise a strategy where they would use Drag0n Slayers to prevent Acnologia from destroying all the dragons. It will no doubt take the power of all Dragon Slayers to defeat him and, if that is indeed the case, then it goes without saying that Gajeel, by himself, would stand absolutely zero chance against Acnologia.
1 Can't Beat: ROGUE CHENEY
In the last showdown between the Iron Dragon Slayer and the Shadow Dragon Slayer, Gajeel managed to come out on top, highlighting his superiority. After a hotly contested battle, Gajeel consumed some of Rogue's magic, turning him into a Shadow Dragon Slayer. These two are oftentimes positioned against each other and it's clear that some sort of rivalry has developed amongst them. Undoubtedly, both are powerful mages that can be proud of their achievements to date but, if one were to pick the stronger of the two, Gajeel would currently come out on top.