She joins from M&C Saatchi where, until earlier this year, she was the European chief operating officer.
In her new role, Gloster will work alongside the founder members, Nick Hurrell, Neil Dawson and Shaun McIlrath, to expand the agency's offering.
She will also take over many of the day-to-day duties of running the business. However, her first job will be to work on the upcoming 3 pitch.
While at M&C Saatchi, she worked with Hurrell and was responsible for launching M&C's offices in Paris, Berlin and Madrid.
Hurrell said: "Sunita will play a crucial role, installing the processes and technical systems we need to service our clients as we grow. When it comes to the running of an agency, there is no-one better. Her experience with international clients will also prepare us well for the future."
Gloster added: "I'm thrilled to be joining Hurrell and Dawson. The opportunity to build a business that is approaching everything with a fresh perspective was an offer I couldn't refuse."
Before moving to the UK, Gloster worked in Australia at Whybin Laurence\TBWA, as well as a stint at the Campaign Palace agency.
When she moved to the UK, she became the director in charge of the Camelot business at WCRS, before taking up a position at Lowe as its worldwide business development director.
Last month, Hurrell and Dawson announced that it would be positioning itself as a full-service agency by offering media planning and buying as well as creative.