Loft played 196 times for Vale between 2009 and 2014, after signing from Brighton, before going on to make 70 appearances for Gillingham.
A statement from Gillingham said he died following a medical emergency over the weekend.
"He will be remembered fondly by everyone at the club, not only for his achievements on the pitch but for the commitment and leadership he showed throughout his time in Burslem," a Port Vale statement read.
"The thoughts and heartfelt condolences of everyone at Port Vale are with Doug's family, friends, former team-mates and all those who knew him, at this incredibly sad time.
"With the blessing of Doug's family, we have requested permission from the EFL for our players to wear black armbands and to hold a minute's silence ahead of kick off at this weekend's game against Chesterfield."
Loft came through the ranks at non-League Hastings United and signed for Brighton in January 2006, just after his 20th birthday.
He played three times in the Championship, scoring once, and made 46 further appearances for the Seagulls in League One.