"Otto vor dem Gentschenfelde" is one of those old German rural names, that just combines a name with a place. Like maybe a Farmer called Otto had his farm in front of the "Gentschen"-field. Maybe there were a bunch of other Ottos closeby so they had to distinguish.

And then it became a last name for his descendants. While googeling I found an obituary for a guy called Dieter Otto vor dem Gentschenfelde, where Dieter was his first name and Otto was part of his last name.