Millions of pounds were said to have been funnelled through travel agencies over six years while "sexual bribery" was also alleged to have been used, according to reports in China.
The gifts were said to have been illegally provided in order to boost sales and raise the price of medicines.
Authorities named four senior GSK executives in relation to the claims, which surfaced last week when officials said suspects had confessed to the crime. At the time, the company said it was willing to co-operate with the probe but that its own monitoring had found no evidence of bribery or corruption.
In a further statement, it said: "We are deeply concerned and disappointed by these serious allegations of fraudulent behaviour and ethical misconduct by certain individuals at the company and third-party agencies.
"Such behaviour would be a clear breach of GSK's systems, governance procedures, values and standards. GSK has zero tolerance for any behaviour of this nature. GSK shares the desire of the Chinese authorities to root out corruption. These allegations are shameful and we regret this has occurred."
The funnelling of money through travel company contracts allegedly helped GSK executives bypass company rules that prevent giving gifts of more than 300 yuan (£32) per recipient.
Gao Feng, head of the economic crimes investigation unit at the ministry of public security, said that since 2007 the company had transferred as much as three billion yuan (£324 million) to more than 700 travel agencies or consultancies, though it was not clear how much was allegedly channelled into bribes, Reuters reported.
"We have sufficient reason to suspect that these transfers were conducted illegally," Gao was quoted as telling reporters. "You could say the travel agencies and GSK were criminal partners. Among the partners, GSK was mainly responsible. In a criminal organisation there is always a leader."