Hate-filled Allen, 47, became the man who literally got away with murder for decades- until now.
Today, he starts a life sentence after being found guilty of the murders of sex workers Samantha Class, 29, in Hull in October 1997 and Alena Grlakova, 38, in Rotherham on Boxing Day 2018.
After having sex with Ms Class, who threatened to report him to police for rape when the condom they were using split, Allen stamped on her, strangled her with a ligature and ran her over with a car, before dumping her body in the River Humber on October 26, 1997. Her body was found by schoolchildren on the foreshore of the Humber estuary.
After dumping her body, Allen returned to his home in Hull and made an early morning trip to the launderette to wash away any evidence. He also sold the car he had picked Ms Class up in to cover his tracks.
It wasn't until he was arrested for drink driving by police in November 1998, that officers matched his DNA profile to that found on Ms Class and he was subsequently charged with her murder.
Allen stood trial at Sheffield Crown Court in January 2000, but was acquitted of the offence.
Upon his release he moved back to the Grimsby area, a move to be the first in his downfall. Aware of his movements and fearing for the safety of sex workers across East Yorkshire, Humberside Police launched Operation Misty.
One of the undercover officers, who was known as Ian, developed a close relationship with Allen over time, telling him about his fictitious involvement in criminality in the Netherlands.
It was a conversation in a pub with 'Ian' one night, that Allen admitted to murdering Ms Class after they had sex and the condom split and she threatened to report him to police. He proudly stated: "I had a bit of string in the back of my car and I said to her 'hang on, I'll just get my wallet and I strangled her'.
While more evidence was needed to charge Allen again, he moved on, to Rotherham, where he met Alena Grlakova at a bus stop on his way to work at a factory.
Ms Grlakova was from Slovakia and had moved to the UK in 2008, settling in Rotherham with her husband and children. Sadly, her marriage ended and she lost contact with her children. Her lifestyle became more chaotic and she had turned to sex work.
On Boxing Day 2018, Ms Grlakova had turned up at Allen's flat twice and both times he had told her angrily to leave because he did not want her there.
In a chilling audio recording that Allen made of their conversation, he told her that if she came back again, he would 'beat her'.
Ms Grlakova was never seen alive again as Allen carried out his threat, strangling her to death. He then went to the shops, bought the tools he needed and concealed her body in the Old Slaugh in Rotherham.
Her body was discovered on April 8, 2019.
Allen came to the attention of South Yorkshire Police after detectives found significant telephone contact between him and Ms Grlakova on her phone before she disappeared.
Forensic analysis of his mobile phone also revealed he had searched for "ways to commit the perfect murder".
Both South Yorkshire Police and Humberside Police have described their investigation into Allen as "hugely complex and challenging".
DCI Mark Oughton, the senior investigating officer for South Yorkshire Police, said: "We were made aware of the undercover recordings by Humberside Police and it was at that point the collaberation between the two forces began. That is when we got legal counsel involved and the Crown Prosecution Service and started looking at double jeopardy legislation to try and get the acquittal overturned.
“The advice from an early stage was that a joint murder trial - for the Rotherham and Hull cases - would give the best chance of a conviction. But that was easier said than done.
"The primary objective was to secure justice for both families, who had been devastated by Gary Allen's actions more than two decades apart. However, the timetable was daunting, and not clear-cut, with gigantic legal hurdles to overcome.
Following a colossal amount of work, the two forces met the Director of Public Prosecutions Max Hill at the Court of Appeal, and successfully overturned Allen’s previous acquittal for Ms Class's murder.
DCI Calvert said: “This was a major moment for Humberside Police, and for a family which had awaited justice for more than 20 years.
"There is never going to be a happy ending in a case like this, and nothing we can do will ever bring either of these women back to their families. However, we’d like to thank all those who have helped to secure Allen's convictions, and hope the families of both Alena and Samantha can take at least some comfort in the fact that justice has finally been served."
The National Crime Agency is now investigating whether Allen is linked to any other crimes across the country.
DCI Oughton said: "Gary Allen is an extremely dangerous individual who is now starting the beginning of a significant life sentence.
"If you look at his offending history over the years, it speaks for itself really.
"We have done extensive work to build up a picture of his life over a number of years, but he has been in prison for a lot of that time following the 2000 attacks.
"He has been constantly monitored by both police forces in Devon and Cornwall and South Yorkshire and Humberside. Having built up that picture, we have liaised closely and sought advice from the National Crime Agency's analytical section. Any potential links that have been found to other crimes will be flagged up to the relevant forces to investigate further."