Hide folder only from stock gallery without using Samsung's private mode
I wanted to do this and the .nomedia file approach is the way to go as it is used by certain apps to hide images that they don't want indexed (such as pics of birds in the birds of Britain app).

However it's a bit of a devil to do. Firstly you must create this .nomedia file in every directory that you don't want indexed, not just the top level one.

The main difficulty is trying to get the gallery app to forget the indexed images. It goes without saying that the next step involves clearing the app's data and cache to force a reindexing (this will also possibly require a reboot of the phone if the gallery app refuses to work afterwards - this happened to me and I had to reboot the phone to get it to work again - sometimes it just worked after clearing it's data, sometimes it didn't - random pot luck). I had to repeat this a couple of times to get it to work. Go to settings -> apps -> all apps -> gallery and tap on stop app, clear cache, clear data.

If you decide to create a new folder and create the .nomedia file inside that and then move the images you don't want indexed into that directory then that should just work as it does for other apps. The difficulty comes when you try and get the gallery app to honour the .nomedia file on directories that it has already indexed.

This worked for me on a Nexus 4.