Ghandruk is a short trek. Its lovely and and just a day walk or less. If you are flying, take a flight to pokhara. Then take a public bus to Baglung or Beni and get off at Nayapul. Or simply you can take any direct bus to Baglung/Beni. You can hire a cab to Nayapul as well. Nayapul is on the way to Baglung/Beni from Pokhara which is located in Pokhara-Baglung Highway. From Nayapul, its almost a day trek to Gandruk. Better get a map or ask someone on the way from time to time because there are a lot of trekking routes around this place so be sure take a right one. From Nayapul you have to walk along the side of Modi River. What I did is we stayed in a hotel in Syauli Bazar( its also a called kolte bhatti) . Then its like 4-5 hrs uphill trek to Ghandruk. Better stay near kolte bhattito so that your treak will be easier. This is the shortest route.