The 28-year-old is a free agent and will look to impress Vale boss Micky Adams in an attempt to earn a deal at the League Two club.
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 signing for Southend in 2004. He moved to Yeovil two years later before a switch to Brisbane Road in 2007.
Gray has been on trial at Rotherham and Exeter over the summer.