The Palo Alto-based company has been testing a beta version since April and will start selling a repackaged product to big clients — who will be charged up to $30 per user per month — from the first week of August.
More from the Financial Times:
KKR makes first move to float First Data
China takes stock as market dust settles
Musk says SpaceX complacent before crash
The software for Symphony, which sprang out of an in-house messaging project at Goldman Sachs, is open source so customers can build their own features on top, unlike the closed Bloomberg system.
"We wanted a way for everybody in an organisation to have a rich communication and collaboration tool," said Mr Gurle. At the moment, there are too many "silos of messaging islands", he said, so people often "default to email, the only universal bridge. But [email] is not rich and powerful and does not build institutional knowledge and it is not social."
Rival companies have tried for years to loosen Bloomberg's grip on messaging, and it is unclear whether a tool backed by multiple banks will flourish where others have failed. At least 320,000 Bloomberg subscribers around the world use Instant Bloomberg, while Thomson Reuters has more than 240,000 users of Eikon Messenger.
Bloomberg declined to comment. A Thomson Reuters spokesperson said the company would "look to work with any partners" who share its goal of "securely connecting the broader community across financial markets".
According to Mr Gurle, the snooping scandal of May 2013 was a "trigger" for banks to review their reliance on Bloomberg terminals for functions including messaging. Then, following a complaint from Goldman, Bloomberg admitted that it had allowed reporters to access information from the terminal — including details of when clients last logged on, what dealings they had with the helpdesk and which of the terminal's functions they were using. Such practices had ended, the company said.
About a year later, Goldman merged its in-house messaging product with a similar tool that Mr Gurle had been developing and rebranded it as Symphony. These days, Goldman has an "equal voice" with 14 other financial investors which have chipped in about $70m of funding, said Mr Gurle.
The full line-up consists of Bank of America Merrill Lynch, BNY Mellon, BlackRock, Citadel, Citigroup, Credit Suisse, Deutsche Bank, Goldman Sachs, HSBC, Jefferies, JPMorgan, Maverick Capital, Morgan Stanley, Nomura and Wells Fargo.
A free version of the Symphony platform will be offered to the public from mid-September, and a cheaper format for smaller companies should follow soon after, said Mr Gurle.
There was no reason that Symphony could not extend into other industries such as healthcare, said Salman Ullah, a partner at Merus Capital, a Palo Alto-based venture firm, and a former colleague of Mr Gurle's at Microsoft. Merus backed Symphony's seed-funding round and is now the only non-Wall Street investor, with about 5 per cent.
"We think this could be a WhatsApp for enterprises," said Mr Ullah, citing the instant-messaging app owned by Facebook. "It is one thing being able to talk to people within a company, but it's another thing to message outside the company, in a completely frictionless and secure way."
He cited an email he had just received from his stockbroker at Fidelity, asking him to check the company's own website for a reply to an inquiry.
"That is hardly state of the art," he said.