Gerrard's strikes either side of halftime put the visitors in control and Luis Suarez sealed only Liverpool's second win at Old Trafford in nearly 10 years with a well-taken third.
Skipper Gerrard could have completed a rare hat-trick of spot kicks but struck the post after Nemanja Vidic was red-carded for tripping Daniel Sturridge shortly before Suarez completed the rout.
Gerrard's first penalty after 33 minutes followed a deliberate handball by Rafael that could have earned the defender a second yellow card, while his second shortly after the break was for a clumsy Phil Jones challenge on Joe Allen.
United's afternoon went from bad to worse when Vidic sent Sturridge sprawling in the area with a lunging tackle and though Gerrard could not beat David de Gea for a third time Suarez's neat finish after 84 minutes capped a dazzling Liverpool display.
Defeat left United languishing in seventh place with 48 points, 11 adrift of fourth-placed Arsenal who are in action later on Sunday at north London rivals Tottenham Hotspur.