I had this very same error with a different plugin (reST). The problem the error was caused by was that I had run it from the command line when avirtual environmentwas active. For this reason Python3 did not use (and find) the system libraries.
Solution: I ran gedit just normally from the GUI (or after deactivating the virtualenv in the Terminal), and the editor and the plugin just loaded fine. Double-check if you have a similar cause.
Otherwise you probably really have to check on what the error message says: Whether all "related packages required by libpeas" are installed. See details of package libpeas-1.0-0 for Trusty.