It’s quite common for manga writers to get inspiration from other authors’ work and share some qualities. This seems to be the case of Fairy Tail which shares a LOT of similarities with One Piece.
For example both Gildarts and Shanks play similar roles in their respective shows. Both men are basically non-biological fathers to the main character and act as mentors and role models. Both men are now primarily absent from the protagonist’s life. And they look the same.
As shown in the following pictures, there are so many similar scenes/character designs between One Piece and Fairy Tail: