Actor Andrew J. West (see his IMDB page) will be joining The Walking Dead cast later in season 4 as a character named “Gareth” and there is an option for him to be promoted to a series regular next season. Details about the character haven’t been given yet. There is no character in the comic book named Gareth but they could be using a fake name for now or this could be a character from the comic book but just given a different name for the show, see quote below.

However, sources tell THR that while Gareth might not be playing someone from the comics, he is a remix of sorts of an unidentified character from the series. Gareth will have what sources say is a big presence and play an important character on the drama from showrunner Scott M. Gimple.
Source: hollywoodreporter.com