GECO is the acronym for a very old German maker... at one time lots German surplus ammo in 8x57mm and 9x19 ammo was on the market... of course now long gone.
If I recall correctly the company name is Gustav Genschow Located in Durlach in the German state of Baden. Here are a couple of photos with boxes of their ammo... probably all pre WWII.
In short GECO is a very old ammo maker who makes top quality stuff... I have shot a lot of it over the years and had good results...
FWIW
Chuck