The worst type of Synthetic you can encounter why? Because they are an entire species cretaed hundreds of thousands of years ago that continually upgrade themselves but what is more disturbing is that their base weaponry can kill Taiidans in a few shots, they do have issues killing the Black Codex II soldiers though.
These Synthetics were created with hatred in the heart of their creators and when you put give a Progenitor a hatred of his creation the Gein is the result. The Gein can kill Taiidans and with ease provided the Taiidan isn't heavily armored and they are the "Demon Hunters".
Created during the Manti Wars they haven't changed in design but they were hunting Taiidans during the Manti conflict because the progenitor who helped create the manti was angry that they were being destroyed.
The beings known as the Gein became feared around the Novan and when they disappeared the galaxy was thankful and had a breath of fresh air. But their return in the Milky Way galaxy was the worst thing that could happen during the Synthetics war. They began taking out the UGI soldiers and saw the MARS as their brethren but couldn't grant their brethren their technology.
AL Reality Edit
In the alternate Reality (Roleplay Timeline B) The Gein are among the races unified with the dreaded Rakshi, and are considered Mild Friendlies to them which is unusal since the Rakshi hate all life even themselves.