Gel, or sometimes known as Kel, is the boss of Chapter 19B: Battle in Bulgar (Sacae) in Fire Emblem: The Binding Blade. He is defending the city of Bulgar in Sacae under orders from Brunnya and is most likely Brunnya's lieutenant. While he is charged with the defense of Bulgar itself, Roartz is put in charge of Bulgar Keep. Compared to Roartz, Gel is brave and not afraid of death. He is very loyal to his superior, Brunnya.
Gel's evasion is nothing short of astronomical, especially on hard mode. Even units who specialize in skill will find themselves dealing with a shaky hit rate. Some axe users might not even be capable of hitting him. His evasion is complemented by a respectable strength stat and the notorious +30 critical boost that Swordmasters in his game come equipped with. He is outright immune to doubling in Hard Mode and stands a very good chance of doubling most units.
Thankfully, Gel's firepower is severely mitigated at a distance. Due to the mechanics of the Light Brand, he is completely incapable of landing Critical Hits at a distance, and his attacks deal a flat 10 damage. Furthermore, Anima magic is effective against the Light Brand, and extremely accurate Anima magic is readily available in the form of Fire and, should the player decide to use it, Forblaze. As long as Gel is left to an Anima mage or archer with sufficient accuracy and the slightest bit of healing aid, he should eventually go down.
Another possible but fairly mindless strategy is to circle around him with various 2 range units on player phase. If their speed ratings are low enough, they can get double struck and receive 20 HP of damage, but this also lows Light Brand durability by to points each. And the damage dealt can be healed until all attacking units have at least 21 HP before player phase ends. His Light Brand can be easily broken in three turns. This will make him vulnerable for an all on assault.
—Gel's death quote in The Binding Blade.