Former Sunderland and Hearts goalkeeper Gordon is without a club and has been training with the Light Blues.
Boss Ally McCoist is keen to strengthen the position after confirming Neil Alexander is unlikely to remain with the Third Division champions next season.
Asked for his thoughts on Gordon, Foster said: “He’s an international goalkeeper.
“Everyone agrees that, when he’s fit and playing, he would be in the Scotland set-up.
“Signing someone like that would be massive.”
Foster agreed a two-year deal with Rangers following his release from Bristol City and will complete his move on September 1 when the club’s transfer embargo is lifted.
SUNDERLAND FIXTURES:
Saturday August 17
15:00Premier League Sunderland v Fulham
Saturday August 24
15:00Premier League Southampton v Sunderland
Saturday August 31
15:00Premier League Crystal Palace v Sunderland
Saturday September 14
15:00Premier League Sunderland v Arsenal
Saturday September 21
15:00Premier League West Brom v Sunderland
Saturday September 28
15:00Premier League Sunderland v Liverpool
Saturday October 5
15:00Premier League Sunderland v Man Utd
Saturday October 19
15:00Premier League Swansea City v Sunderland
Saturday October 26
15:00Premier League Sunderland v Newcastle United