The visitors were given a warning early on when he almost gave Millwall a dream start. Byfield pounced on a pass from Poul Hubertz and burst clear of the Gillingham defence only to see his fierce drive tipped around the post by Dean Brill.
Despite having the better of the opening exchanges, the home side found themselves a goal down on ten minutes through Gary Mulligan.
Bas Savage flicked on a long throw to the edge of the six-yard box and his strike partner swivelled and smacked the ball into the net with an excellent volley.
Midway through the half Chris Zebroski replaced Chris Hackett on the right wing and within ten minutes the substitute had his side on level terms.
Neal Ardley swung in a searching cross which the Gillingham defence failed to clear and Zebroski was first to react from 12 yards out by thumping the ball past Brill on 38 minutes.
After the break the home side continued where they left off and were in the lead just minutes after the restart.
Darren Jupp brought down Byfield with a clumsy challenge on the edge of the box and referee Keith Stroud had no hesitation in pointing to the spot.
Byfield picked himself up and fired the penalty past Brill to give his side a deserved lead.
The goal finally fired Gillingham into action and Andrew Crofts produced a magnificent save from Lenny Pidgeley with a tremendous drive from distance that the former Chelsea man tipped around the post.
Gillingham almost equalised from the corner when Marvin Elliott headed the ball towards his own goal but was saved by a diving clearance off the line from Marvin Williams.
Byfield wrapped things up with ten minutes to go when he beat the offside trap and slotted the ball past the helpless Brill with a cool finish before latching on to a defensive error from Ben Chorley to seal his hat-trick.