Based at Rhu Marina and with operations in Holland and Australia, GSS will move its base to the Silverhills boatyard in Rosneath, which dates back to the 1880s, after buying the property, stock and assets.
It said: "This will bring to a successful conclusion to GSS' nine year search for permanent premises."
Jamie McGarry, managing director, said: "We were constrained by the need to stay local to the MoD in Helensburgh. We were in Rhu Marina with great support from the Crown Estate but this opportunity was too good."
Mr McGarry said the business was bought as a going concern and the brand as well as all the staff would be retained.
"It has a hugely talented team of people and it our absolute intention is to turn it into a centre of excellence as a commercial and leisure marine facility," he said.
Since 2010 GSS has set up in Holland and Australia to service its client base, and it employs more than 150 in its charter fleet, marine civil engineering, facilities management and diving divisions. Its vessels are currently working in the dredging, cable-laying and oil and gas sectors across the globe, the company said.
Mr McGarry would not comment on the company's financial metrics but said: "We just want to continue on the steady and measured growth path we are on."
Accounts filed at Companies House show that the business turned over £20m last year making almost £2m pre-tax profit. A second business at Rhu, Gareloch Support Services where Mr McGarry is sole director, turned over almost £19m and made a pre-tax profit of £1.9m.