I heard live from HaGaon HaRav Yis'haq Yosef Shelita (author of Yalkut Yosef) that it is Mutar. He also said candies are Mutar in the bathroom.
Just for additional information, The Sepharadi Rabbi I asked (R' Shay Tahan) said that if the gum still has taste, then it's assur to keep it in one's mouth when going to the bathroom.
This Ashkenazi site says that it is assur.
This website says that it is נכון מאוד (very much proper) to not have the benefit of the taste from gum in the bathroom.