According to the Institut Géographique National from 1 January 2007 until July 2013 the geographic centre of the European Union was located on a wheat field outside the town.

The emperor also granted trade privileges like the staple right which forced traveling merchants to offer their goods in the town for three days.

However prosperity came to an end already in 1326 when Emperor Louis IV gave the town in pawn to the counts of Hanau, redeemed shortly afterwards.

Repeated plundering in the Thirty Years' War as depicted by Hans Jakob Christoffel von Grimmelshausen in his novel Simplicius Simplicissimus made it nearly uninhabitable.

The varying lords made continued attempts to challenge Gelnhausen's imperial immediacy, it however formally remained a Reichsstadt.

During the Nazi era, Gelnhausen was reported judenfrei on November 1, 1938, by propaganda newspaper Kinzigwacht after its synagogue was closed and remaining local Jews forced to leave the town.