So the Gutterman lore terminal entry says that it was an experiment with blood fuelled machinery, but they couldn't keep the blood inside fresh so they stuck a live human in there before welding it shut. Very frequently it was unwilling.
Obviously, it'd be a terrible idea to let the world know that you're sticking live human beings inside your war machine for the purpose of draining their blood, so the government that created them (implied to be Russia) stated that those inside were willing patriots. They weren't, they were enemy soldiers, fatigued war vets, and criminals fairly often.
After the Guttermen made trench warfare obsolete, the Guttertanks were developed. Based on the Gutterman to save resources, it had a mean right hook and a rocket launcher that it could remotely detonate. Due to these, the shields of the Guttermen were no longer useful. Based on the writing on the arm of the Guttertank, it is implied that Germany made them. The creation of these Guttertanks is what properly kickstarted the ~200 year arms race that was the Final War.