The move is a blow for College Hill Associates, which has advised GWR for the past ten years.
Finsbury's relationship with Capital Radio is also established but stretches back just two years.
GCap communications director Jane Wilson, who until this week held the same role at Capital Radio, said Finsbury had fought off College Hill and an external challenge from The Hogarth Partnership.
College Hill chairman Alex Sandberg commented: 'This is a merger - some you win, some you lose.'
Wilson joined Capital Radio in 2003 from Impact Consulting shortly after the agency was acquired by Mantra PR. With no opposite number at GWR, Wilson has been given the top comms post and runs a six-strong PR team.
Capital Radio and GWR announced their merger last September, creating the UK's largest commercial radio group, which now reaches 17 million listeners by pooling 55 local analogue and 100 digital stations.
The group controls around 40 per cent of the UK radio advertising market and owns the Capital FM, Classic FM and Xfm brands.
But on Monday GCap's first day of trading was marred as it revealed a 17 per cent decline in the group's April revenues, owing to weaker consumer confidence and low ad spend.
GCap's appointment of Finsbury follows Heart 106.2 owner Chrysalis's decision to part company with Buchanan after ten years (PRWeek, 18 February).
Chrysalis, which subsequently hired Financial Dynamics, had been struggling for audience share against Capital.
But last week Heart overtook Capital's as London's most popular station.
- What the papers say, p16.