The teenager was within the 12th floor stairwell of a multi-store car park in Oswald Street with a 17-year-old boy when she was sexually assaulted.
The incident happened at around 11am on Tuesday, July 29.
Detectives are keen to trace two people who they believe may assist with the ongoing enquiry.
A dark haired woman carrying a quantity of books was seen entering the landing just before the girl was assaulted, and shortly after the incident, a man with dark hair and wearing a dark suit passed them on the stairwell.
Detective Inspector Craig Willison, of Greater Glasgow Division's CID, said: "We are keen to trace these two witnesses, who potentially could provide us with information which may help with our enquiry.
"I'd like to reassure the public that we are following a positive line of enquiry in this case, and I would like to reiterate that Police Scotland takes any report of any sexual assault very seriously.
"If you are a victim, please contact us and we will provide you with the support you need."
Anyone with information is asked to contact police on telephone number 101 or alternatively Crimestoppers on 0800 555 111 where anonymity can be maintained.