The 28-year-old is a free agent, he was at Leyton Orient last season but was released in the summer, so now he will be looking to impress Vale boss Micky Adams as he tries to earn a contract with the Potteries outfit.
Gray enjoyed a month-long loan spell at Vale Park back in 2000 when he made three appearances without scoring.
He also spent time on loan elsewhere during his rise through the ranks at Wimbledon (now MK Dons) before he went off to Southend in 2004.
He moved to Yeovil two years later before a switch to Brisbane Road in 2007.