It was the second victory in as many games for the struggling defending champion, who stayed ninth on the ladder six points from the top six, and its first ever against United to ease the pressure on under-fire coach Vitezslav Lavicka.
Petratos, who turned 18 last November, slammed the ball into the top right-hand corner from the edge of the box in the 32nd minute for a memorable first goal of his career, after latching on to an exquisite pass from Hirofumi Moriyasu.
The goal was just reward for Sydney's dominance in the opening period, with marquee signing Nick Carle and Japanese schemer Moriyasu bossing a Gold Coast midfield missing Australia star Jason Culina, who is on AFC Asian Cup duty in Qatar.
United started well and should have taken the lead in the 13th minute through Zenon Caravella after Robson combined with Adama Traore to outpace Stephan Keller but the midfielder rolled his shot past the post in what proved to be his side's only genuine chance.
Carle and Moriyasu both spurned decent opportunities before hard-working Petratos broke the deadlock to give the disappointing crowd of 6135 something to cheer about.
United came into the game more in the second half but former Socceroo Bruce Djite and Chris Harold were ineffectual in attack and the return of New Zealand striker Shane Smeltz to the club following a miserable spell in Turkey cannot come quick enough with a four-game winless streak threatening to derail its push for a top-two finish.
Smeltz scored 19 goals in 26 games for United last season and is set to re-sign on a marquee guest contract after falling out of favour with Genclerbirligi.
Miron Bleiberg's side's task was made harder in the 65th minute when veteran right-back Steve Fitzsimmons saw red for a clumsy challenge on Moriyasu.
Lavicka's introduced Makela for his home debut for the final 15 minutes and the 27-year-old Finn should have marked it with a goal when he rose unmarked at the back post to meet a teasing cross from Scott Jamieson, but could only nod the ball wide.
The big striker did find the net in the final minute when he was put through by Sebastian Ryall, and rolled the ball past Moss.
There were worrying scenes behind the goal when defender Matt Jurman's foot got caught underneath an advertising hoarding that collapsed in the celebrations in front of the Sydney fans.
Luckily for Jurman he was able to rejoin the action.