He was assaulted and threatened at knifepoint during the incident at Ross House in the Mount Vernon area at about 7.45 pm on Sunday.
The victim’s sister, who turned up at the flat, was held in a bedroom but was unhurt.
The gang were described as being in their 30s and wearing hooded tops. They spoke with local accents.
Detective Inspector Peter Mullan appealed for anyone with information to contact officers at Musgrave police station on 101.
Alternatively information can be passed to police through Crimestoppers on 0800 555 111.