They feature bizarre body shapes, with 2 spines and a long neck, plus large feet and hands relative to the small, thin torso. The biggest feature is the multitude of Z-shaped bent rail joints. In the story, these are called Twin Swing, and are the main source of GTMs' high combat capabilities, along with the Helios Armor & Harmoid Energy System.
When Nagano's Five Star Stories manga serialization returned in 2013 after a 9-year hiatus, he decided to replace all of the Mortar Headds in the series with GTM. Each type of MH was replaced with its respective GTM counterpart, and the MH, and its predecessor mecha called Machine Messaih, were completely purged from the FSS universe, history, and stories.
Furthermore, "Headliners", the formal, obsolete name of MH's knights(pilots), and "Fatimas" were renamed to "Warcasters" and "AF (Automatic Flowers)", though the previous names remained as aliases. GTMs are about one third larger than the MHs, and their technology and engine output are completely different.
