Saturday night's fight at the Hartwell Arena offered Gammer another chance to kick-start his career.
Former champion Williams was meant to have been the first defence for the undefeated Gammer.
Gammer, 30, rejected a "substantial offer" to fight Harrison in London on 17 February.
"I am going to carry on, I was happy with the way I boxed, " Gammer said.
Gammer was always in control against the Steeds and easily won the judges' decision 120-108, 118-110, 118-110.
The fourth defeat of Gammer's career proved to be the last and now ends his time in the ring.
"The likes of Audley Harrison, Matt Skelton, Danny Williams - they're a lot bigger names than I am, " Gammer said.
The venue can only hold 1, 200 fans, but promoter Paul Boyce wanted Gammer close to his west Wales fan base.
However, Gammer opted to come out of retirement to fight Francesco Pianeta for the vacant EBU-EU heavyweight belt in July 2008.
There were plenty at ringside who thought that Gammer had probably done just enough to edge it, although it was tight.
But early in the sixth a right to the body floored Gammer again and Helenius finished him off with two head shots.
But despite losing his opening-round clash to the night's surprise package, Coleman Barrett, Gammer says he has rediscovered his passion for boxing.
Pembroke Dock heavyweight prospect Scott Gammer scored the best win of his career with a second-round stoppage of experienced Pole Roman Bugaj .
Heavy body shots put Gammer down in the first round, although he got up to battle his way back against 'The Nordic Nightmare'.
Gammer impressed in the early rounds against the German-based Italian but then injured his right hand and was forced to throw in the towel.
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.
"Hopefully we can start working on a date now with Danny Williams, but I want to get it before Christmas if we can, " Gammer added.
Gammer's manager, Paul Boyce, confirmed that the Welshman's career is over.
Gammer is still someway off earning another chance at the title, although his manager Paul Boyce confirmed he will be back in action again soon.
Gammer's absence from the ring showed in the first round at the 5, 000 sell-out ExCel Arena, the Welshman struggling to find his timing against Barrett.
But at 32 years old the Welshman should now be in his prime as a heavyweight and Gammer says this time around things will be different.
Gammer became British heavyweight champion in June 2006 with a ninth-round stoppage of Mark Krence, joining an elite band of seven Welshmen who have held the title.
But Gammer lost his British title to Danny Williams in March 2007 and decided to quit the following September after losing to John McDermott on his comeback trail.
But that did not stop the 26-year-old Irishman from unleashing regular flurries of punches, most caught on Gammer's gloves or dodged but still enough finding a scoring mark.
The first of seven fights on the night pitted Gammer, who had weighed in at 17st 11lb, against Barrett - the lightest fighter on show at 15st 13lb.
Barrett emerged for the third seemingly with a new respect for the power Gammer carried, especially that left hook that kept finding its way through the southpaw guard.
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.
"I felt in control from the first round onwards - there were a few close rounds in there but I still felt I was winning every one, " Gammer said.
Gammer has retired after each of his past three defeats, although the Pembroke Dock man has admitted that his heart was never truly in it when he made previous comebacks.
Barrett, the best of the lot and a worthy finalist after fine victories over Scott Gammer and Carl Baker, troubled Harrison with worrying regularity before getting caught with a sweet left.
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