Ustad ji, I think Gupha-Pokhari is in the southern part of Sankhuwasabha towards Taplejung side. It lies on the Kanchanjungha trekking route from Basantpur which is some hours drive from Dhankuta. It may be around one-and-half days trek both from Basantpur or from Tumlingtar airport in Sankhuwasabha. Once I'd been to Tumlingtar and thought of going to this place but felt it too far.. just visited Manakamana of the east (remember the song - 'wari jamuna pari jamuna.. phedaima manakamana') and came back... This photo by Prem Banem is really impressive, makes me feel Gupha-pokhari and Milke danda are nice places to visit.

Here's the route from Tumlingtar