Have you tried deleting the whole game file and then redownloading this one so no previous files are still present in the new one?
Losing your progress isn't that much of a problem unless you're the type that wants everything to be the way it was before an update, like the location of your last save and such.
If you only want to do the new update, start a new game then do the Christmas update. After your done, edit the save file using any text editor, notepad, what have you and next to a "scene" word and a number, change the number to 40/41. That should be the last stage of the vanilla game and you only need to get to the end of it to reunlock your gallery
IF YOU STARTED A NEW GAME and are having trouble without a gun for the Christmas stage, edit the save file and find the words "isgun", "isrifle", "getGunAndKye", and "getRifle". Those should have the words "false" next to them if you don't have them yet, change them to true and you'll have them when you reload the save.
Gun is for the pistol, rifle is for the rifle