Video report by Jonny Brown
A man has been found guilty at Sheffield Crown Court of murdering two women more than 20 years apart in Hull and Rotherham.
Allen was originally tried for the murder of Samantha Class in 2000 after being pulled over on an unrelated driving offence.
His DNA and fingerprints were found on Samantha's body and he admitted having sex with her and scrapping his car in the days after her murder, but was acquitted at trial.
Just months after Allen was cleared 21 years ago, he attacked two sex workers separately in Plymouth and was convicted of both assaults.
His acquittal for murder was overturned in 2019 by the Court of Appeal after a range of new evidence was presented - including the murder of Ms Grlakova and a confession made by Allen during an undercover operation by Humberside Police between 2010 and 2012.
Police forces around the UK will now be asked to check on unsolved cases to make sure Allen - who has professed a deep-seated hatred for women, and sex workers in particular - has not committed any further offences.
Although Allen spent the best part of a decade in prison following the Plymouth attacks, he was free until he was arrested over Ms Grlakova's disappearance in 2019 and was known to have moved around the UK.
Detective Chief Inspector Mark Oughton, who led the investigation into Ms Grlakova's killing for South Yorkshire Police said: "This is a concern for us too.
"We've mapped Gary Allen's movements when he's been at liberty throughout that period and we've worked closely with the National Crime Agency's analytical section to find out what other offences were in those areas at that time.
"What we're now going to do is to write to individual forces to make them aware that Gary Allen was in the area at that time and ask them to review any crimes that were in that area at that time."
Mr Oughton said: "I think Gary Allen's a very dangerous individual and I think he would have gone on to attack other people if he had not been convicted today."
Mr Oughton said the inquiry began as an investigation into Ms Grlakova's disappearance, which was reported in early January 2019.
And he said his team recovered 37,000 hours of footage and investigated 125 sightings.
He said he became extremely concerned when it became clear Ms Grlakova had had "significant contact" with Allen.
Mr Oughton said his concern grew in April 2019 when his team found "very disturbing, very chilling" voice recordings capturing Allen threatening Ms Grlakova, and he declared it a murder inquiry.
Ms Grlakova's body was found in a Rotherham stream that April - four months after she went missing - and questions have been raised about why it took police so long to find the body.
Mr Oughton said the area had been searched but not the watercourse and it was very well concealed in the stream bed.
Mr Oughton added: "I welcome the conviction today of Gary Allen but my thoughts, the thoughts of South Yorkshire Police and Humberside Police are with the two families.
"Alena's family have had to wait two-and-a-half years for justice. Samantha Class's family have had to over two decades."