In the Treaty Yug (the Silver Age), Sushena (the expert physician in the Vanara Sena that followed Ram) had suggested Vir Hanuman bring Bisalyakarani ere dawn so that Laxman would rise back to life. It was in the middle of the war between Lord Ram and Ravan. Hanuman failed to identify the particular herb and carried on his shoulders a huge Himalayan mass. While flying above and proceeding toward Lanka (the kingdom of Ravan), a portion dropped down. Gandhamardhan Hills is synonymous with that portion only. On the northern slope of this hill, the Nursinghanath Temple is located. Whereas on the southern slope of this hill is the famous Harishankar Temple. Gandhamardhan Parvat is one of the famous hill stations in Odisha. (Source)