Gately and partner Andy Cowles had invited a Bulgarian called Giorgio back to their £1million Majorcan holiday home after they met at a nightclub in Palma on Friday night.
The 33-year-old singer is understood to have fallen asleep on the sofa but is thought to have died after choking on his own vomit following a drinking session.
He was found by his distraught partner on Saturday afternoon but it is unclear whether Giorgio had left the flat before Gately died.
Police believe the Bulgarian could help them unravel the mystery surrounding the death.
A friend of Mr Cowles, who married Gately in a civil ceremony in 2006, said he was in a 'terrible state'.
‘Andy was alone in the flat when he discovered Stephen's body,' she said. 'It was absolutely horrific.'
Mr Cowles made a desperate attempt to revive Gately - giving him mouth-to-mouth resuscitation.
But Gately, who was wearing pyjamas, had drowned from the fluid on his lungs.
Friends said Mr Cowles was in a terrible state and told them: 'I can't believe he's gone.'
Gately was discovered at 1.45pm on Saturday in the apartment in the exclusive resort of Port Andratx in south west Majorca.