GECO- was a firearms retailer in Berlin. Gustave Genshow & Company.
Just as many companies made guns for Sears, several makers made guns for GECO. Most commonly, J P Sauer und Sons. But there were others (Simpson is one). Some were made in Germany, some in Belgium.
Value? Sorry, bro- value of ANY gun is based on exact make, model, and condition. You really need a hands on appraisal from someone that knows Euro shotguns.
Marking on the water table and barrels tell a lot about these guns. PLEASE do not shoot ANYTHING in your gun until a smith checks the chamber length. Length of older Euro shells was shorter than current standard. Just because it fits into the chamber does NOT mean safe to shoot.
In the meantime-