The integrated communications company said the investment would be used to open several new international offices and fund a series of acquisitions in the PR, data management and digital sectors.
The change of shareholder sees the syndicate of investors led by venture capital company Octopus exit the company.
The agency's senior management structure, which is led by Gary Brine, chief executive officer, will not be affected by the new investors.
The news follows the company's acquisition of specialist healthcare PR firm HSD Communications in December, and healthcare marketing agency Woolley Pau in November.
Brine said: "By continuing to expand Gyro International into sectors that offer opportunity for growth and in key geographical markets we are able to offer our clients a truly integrated, international offering."
Gyro currently has offices in London, Manchester, Geneva, San Francisco, New Jersey, Stockholm, Amsterdam, Dublin, Hamburg, Dubai, Madrid and Paris.