John,
"Geco" is for Gustav Genschow & Co,Aktiengesellschaft; a supplier of various things, including guns and ammunition.

Tom,
Since Axel hasn't weighed in yet, I'll say your rifle was made from a WW1 surplus Kar 98.Post war,most sporting mauser rifles were made from old military actions rather than more expensive commercial Mauser actions.Many of these actions had the military markings removed, and some had other markings applied.
Mike