Gray, 47, a former Ulster Defence Association commander in Belfast, was shot dead at point blank range outside his home last night.
Detectives hunting for the assassins said six arrests were made during searches.
The suspects were being held at the Serious Crime Suite in Antrim police station.
Superintendent George Hamilton, the officer in charge of the inquiry, also confirmed that Gray's former associates were believed to have carried out the killing.
He said: "A significant and major line of inquiry is that Mr Gray was murdered by the UDA, an organisation with which he had an association in the past."
The flamboyant ex-paramilitary leader was gunned down outside his father's house in Knockwood Park in the east of the city where he had been ordered to live since being released from prison on bail.
The killers are believed to have been lying in wait for Gray and attacked as soon as he got out of his silver car.
Police believe Gray could have been set up for assassination ever since he was kicked out of the UDA in a ruthless coup against his shameless drug dealing and racketeering in March.
Weeks later he was under arrest as part of a major inquiry into an alleged money laundering scam, and Mr Hamilton confirmed that the deposed loyalist godfather was warned about his safety on several occasions.
"Since Mr Gray's arrest and retention in Maghaberry Prison in April, there has been a number of threats made against him," he said.
"Police have complied with our obligation in terms of advising him about these and given advice about personal security.
"Those threats were given to him while in prison and since his release."
But the detective denied that Gray was under police surveillance.
"Nor was he the subject of dedicated police protection," he insisted.
Mr Hamilton added that officers had gone to his home on occasions to check he was keeping to his bail conditions.
Gray was shot several times in the upper body and died at the scene just after 8pm yesterday.
Despite his notorious reputation, police pledged to try to hunt down the murder gang.
Mr Hamilton sad: "I want to be very clear about this, Mr Gray was murdered.
"This was the brutal killing of a man by another human being.
"The Police Service of Northern Ireland will fulfil its obligation in attempting to bring to justice those responsible for the murder of James Gray."Reuse content