Explanation

The Gomal rises in Afghanistan (Paktika province, in the Sulaiman range) and joins the Indus in Pakistan, so it originates outside the Himalayas. The Gilgit, Hunza and Zaskar are all Himalayan/Karakoram tributaries — NCERT lists the Gilgit among the Indus's Himalayan tributaries. The Gomal is therefore the one that does not originate in the Himalayas.