But early in the sixth a right to the body floored Gammer again and Helenius finished him off with two head shots.
BBC: Gammer retires following defeat
The Helenius-Gammer fight was set to take place on the undercard of the WBA heavyweight title bout between holder Nikolay Valuev and Ruslan Chagaev.
Finnish champions MyPa will be missing star strikers Saku Puhakainen and Niki Helenius in their first-round games set for 12 July and 19 July.
BBC: SPORT | Football | Europe | Striker crisis hits TNS foes MyPa
But the challenger failed a medical on the eve of the fight and it was left to Helenius to take the plaudits as the main attraction.
But the loss to Helenius, 25, a silver-medallist at the 2006 European Amateur Championships, saw Gammer slump to a 18-4-1 record and again question his future in the sport.
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