Gmail Labels Synchroniser
What Does It Do?
Basically, what Gmail Labels Synchroniser does is, as its name suggests, synchronise 2 or more in Gmail. It makes sure that the messages are always the same in the two labels/folders.
Why Do I Need It
Well, for a variety of reasons. The most obvious one would be to make sure you get the same experience with mobile using email apps like Mailbox, which only syncs a few labels. Say you had a label "Mailbox/Urgent", which is accessible through Mailbox easily but troublesome to access from a computer. You could set it up to sync with the label urgent. There are, of course, many other uses for this.
Installation
The installation is a piece of cake! As long as you have a Google account, just make a copy of this script by clicking here and click File>Make a Copy, which will make a copy of it in your Google Drive. Put it anywhere you want.
After that, follow the instructions in the file (in red) CAREFULLY and EXACTLY.