Spotting

I put the link to the Ebird/Clements checklist in with my suggestion, it is a worthwhile download to keep handy and bring you right up-to-date with the latest changes. It is worth noting that a few authorities may disagree on a few counts but it is kept up-to-date and there have been so many changes in recent years due to expanding knowledge, use of DNA analysis, etc., that most books become out of date soon after publication. Common names also vary from country to country so the use of Scientific names is essential for accuracy. If you want to call it Heuglin's Gull or Siberian Gull go ahead, but Larus fuscus gets it to species level and heuglini is the scientific sub-species name - at the moment.

Full list for Nepal and you can rule out the regulars.
Mew gull, Larus canus (A)
Herring gull, Larus argentatus (A)
Lesser black-backed gull, Larus fuscus (A)
Heuglin's gull, Larus heuglini (A)
Caspian gull, Larus cachinnans (A)
Great black-headed gull, Larus ichthyaetus
Brown-headed gull, Larus brunnicephalus
Black-headed gull, Larus ridibundus
Slender-billed gull, Larus genei
All the others should have white spots visible on the tails in January.
Yellow-legged and Caspian were one and the same until a recent split made Yellow-legged the Western Europe/Atlantic species, now L. michahellis.
L. icthyaetus in non-breeding would still have dark patches behind the eye.
Mew is too small (nearer size of small Mallard), Herring and Caspian have large white spots on folded primaries. That only leaves Lesser Black-backed and Heuglin's.
Now for the crunch. The latest move on Gulls has downgraded Heuglin's to a sub-species of Lesser Black-backed, so that makes this a Lesser Black-backed Gull, Larus fuscus heuglini.