Riku came up with the name himself, and is derived from his motto “Sitting around doing nothing will get us nowhere! (ジーッとしててもドーにもならねえ！ Jīttoshite temo dōnimo naranē!)”, though it is confirmed later in the series that his name comes from the words “Gene” and “Destiny”, with the D and E in “Destiny” swapping places.
His name could also be derived from the words “Gene” (ジーン Jīn) and “Seed” (シード Shīdo). The word “Gene” refers to the DNA of each Ultras respectively, contained within the Ultra Capsules. Alternatively, as the cloned son of Belial, he bears nearly identical genes to his father. For ‘seed’, this may refer to how Geed was grown from Belial’s genetic material. In addition to this, Geed as Riku spent many years not knowing of his past, until RE.M. ‘germinated’ his knowledge of his true self.