Gilbey then came within inches of finding a dramatic equaliser only to strike the post in the closing stages, before Nouha Dicko made sure of the points in the dying moments.
Kieran Agard's first-half brace looked enough for the Millers to down the promotion-chasing Bees, before Haris Vuckic pounced in the final moments to cap an impressive win as Steve Evans's men remain comfortably in the playoff spots.
Neil Kilkenny cancelled out Danny Swanson's opener to earn a 1-1 half-time draw for the Lilywhites, before two Joe Garner strikes in the final 10 minutes were enough to see off Posh, who now sit 14 points adrift of fifth-placed Preston.
Murray struck once more in the final few minutes of normal time before Mark Ellis bagged a consolation for the Railwaymen late on.
Consequently, Shaun Derry's men move off the bottom of the standings up to 23rd, one point off safety.
Results in full: Bradford City 0-2 Walsall, Bristol City 5-0 Port Vale, Carlisle United 0-0 Shrewsbury Town, Crawley Town 0-2 Sheffield United, Crewe 1-3 Notts County, Leyton Orient 1-1 Oldham Athletic, MK Dons 0-1 Gillingham, Preston North End 3-1 Peterborough United, Rotherham 3-0 Brentford, Tranmere 1-2 Swindon Town, Wolverhampton Wanderers 4-2 Colchester United