Since you already have a green card from your refugee status, you only need either a re-entry permit valid for 2 years or refugee travel document valid for 1 year. You can visit nepal with no problem unlike visiting Bhutan. The only thing you need to be aware is that, some countries do not recognize refugee travel document on transit like UAE; So, you need to avoid transit in Dubai. You can go via Qatar, Singapore, London, France, Germany with your RTD or Re-entry permit with no problem. To be 100% sure, get your ticket and confirm with the consulate of the transit country whether you can travel via their airport with your travel documents. Have fun in Nepal.