The attacker had consecutive loan spells at Valley Parade as a Newcastle United youngster before spells with Shrewsbury Town and Scunthorpe United, where he wore the captain's armband at times last season.
“I am over the moon to be back here," said the 25-year-old, whose Glanford Park contract expired in the summer. "I have spoken to the manager and the rest of the staff, and it is an exciting project which is being built.
“I have been here when we have been doing well, and when we have not really been doing so well, but this is a great club and I am looking forward to getting going again.
“The manager is a proven one at this level, having achieved what he has over the past few years, and what he had to say over the phone to me was very encouraging and exciting.
“I feel like I have grown up a lot as a player and as a person since my last spell here. I have a lot of different attributes now, and want to show that to the supporters.”
Gillead has signed a two-year contract with the League Two side.
“He is a talented player with the ability to change games and unlock doors, which will be crucial for us in our pursuit of winning matches next season and beyond," said manager Derek Adams
“Having watched him closely at Scunthorpe for the past couple of years, I am well aware of what he will add to the group. On top of bags of ability, Alex also has a great personality and leadership traits, which are always what we look for in players."