* Changing Hair Styles: You can change your character's hair style (and color) at the Barber in Battleon. This costs Adventure Coins (ACs), the premium currency. You can't specifically get hair gel; instead, you buy a new hairstyle that might look like it uses hair gel.
* Armor/Items with "Gel" in the Name: There might be armor, helms, or other items in the game that have "gel" or "slick" in their name, implying a certain hairstyle. These items are obtained through quests, shops, or drops from monsters, depending on the specific item.
If you're looking for a specific item or style, let me know the name, and I'll try to find out how to obtain it.