Turner, who is the reigning Commonwealth and European 110m hurdles champion, was allegedly promised a £132,000 three-year contract and a £15,000 Audi car as part of the deal.
It was said Cas wanted Turner as a ‘big name’ in order to entice other athletes to GSG.
But a jury at Croydon Crown Court found Cas (47), of Thornton Heath, Croydon, not guilty of conning Turner and cleared him of two other charges.
However Cas, who had already admitted seven specimen charges of fraud, was convicted of two more counts.
He conned Olympic gold-medallist sprinter Mark Lewis-Francis with a three-year deal worth £144,000 and heptathlete and former ‘Gladiators’ TV star Lucy Boggis, who agreed an eight-year contract with GSG worth a total of £465,000. Cas was jailed for three years.
In passing sentence, Judge Nicholas Ainley told him: “You decided you needed big names to reel in smaller fish. This was a very nasty fraud.
“You decided this business was not going to make enough money for your greedy purposes so you swindled people.”
The GSG website claimed to have £35 million in backing from high-profile sponsors such as Vodafone, Virgin Airways and Audi, and suggested he could match firms interested in sponsoring sportspeople with up-and-coming athletes.
The scam was targeted to take advantage of the up-coming Olympic Games in London next year.
Det Con Nichola Lester, of the fraud unit in Croydon, who investogated the case, said: “Throughout the investigation, Cas has shown no remorse for his crimes.”