Geco was the trademark for Gustav Genschow & Co., a German ammunition maker, but also a maker of .22 rifles. The guns are of good quality, but the only one I know of with any substantial value is the .22 caliber (Kleinkaliber - "KK") copy of the Mauser K.98k which was used by the Hitler Youth and others as a training rifle.
I think Geco is still in business, though in ammunition manufacture only.
As a guess, I would say that unless it is the K.98k copy, the rifle is worth in the $150-200 range. The trainer version would bring about $400.
Jim