Gilliead, 20, penned his loan deal with City this Thursday before training with his new team-mates at the Club's Apperley Bridge training ground.
The 6' 1'' attacker impressed earlier this season after spending the first half of the 2016/2017 campaign on loan at Luton Town.
Born in County Durham, Gilliead also featured 41 times with Carlisle last season while on loan at Brunton Park.
Gilliead, who turned professional with Newcastle back in 2013, has been capped at U16, U17, U18 and U20 level for England.
Subject to receiving the relevant clearances, Gilliead could now make up part of City boss Stuart McCall's squad for this Saturday's trip to Shrewsbury Town.
Gilliead will wear squad number 18 with City for the remainder of the season.
Speaking about the move, McCall said: "Alex is someone we have been monitoring since the summer.
"We feel that we need a bit more competition in the wide areas, and Alex will certainly give us that.
"He is strong and quick, and he's had two good loans under his belt at Carlisle and Luton.
"He is fresh and positive as a player, and hungry to succeed.
"He has trained with the squad today (Thursday) and, all being well, will be in the squad with us travelling down to Shrewsbury Town this weekend."
Gilliead himself said: "I am buzzing to be here at such a big club.
"I came to watch the game last Saturday (vs Chesterfield) and I saw the size of the crowd and the way the team played. I was raring to go after that.
"The team's style seems to suit me. They like to get the ball wide and get crosses into the box, and that is what I am all about really.
"I like to get on the ball, run with it, and get at defenders one-on-one. I try to get crosses in and chip in with the odd goal myself.
"I know am I going to have to work hard to get into this team but I want to knuckle down and show the manager, the fans and the players what I'm all about.
"I am looking forward to testing myself at this level."