The Free State outfit left Tshwane with all three points despite defending for the majority of the game.
Both sides were clearly on the offensive from the start as they looked to grab a quick goal and give themselves an early advantage in the crucial clash.
It was Sundowns, though, who were the only ones to maintain pressure for the remainder of the tie.
Elias Pelembe and Jabulani Shongwe went the closest for the hosts in the early exchanges.
First the Mozambican winger managed to get on the end of a cross from Tebogo Langerman in the 14th minute, but the former SuperSport United man only managed to blaze his effort wide of Patrick Tignyemb’s left hand post.
Shongwe then had a chance in the 22nd minute. Nyasha Mushekwi played the ball into the path of Shongwe, who was in acres of space inside the area, but the midfielder seemed to have too much time and totally miscued his resulting effort.
Despite the home side dominating and creating chance after chance, it was the away side who opened the scoring, completely against the run of play.
Gamildien tapped into an empty net in the 38th minute after Wanye Sandilands and Alje Schut clashed in the area. The Siwelele midfielder punished the veteran duo to bag his third goal of the campaign.
The home side dominated the first half but the likes of Langerman, Pelembe, Henyekane and especially Shongwe were unable to take their chances. Celtic capitalised on the chance handed to them by the Brazilians and as a result it was them who went into the break a goal to the good.
The second stanza started off slowly as Phunya Sele Sele sat back and seemed happy to guard their one goal advantage.
Sundowns were struggling to break down the stubborn Celtic defence and as a result Pitso Mosimane brought Katlego Mphela, Edward Manqele and Eleazar Rodgers into the fold as he desperately went in search of the equaliser in the final 15 minutes of the clash.
Edward Manqele and Hlompho Kekana went close for the home side in the 78th minute. The former Free State Stars man beat Tignyemb but his effort was cleared off the line by John Arwuah, before Kekana forced a good save out of the Cameroonian shot stopper.
Despite having four strikers on the pitch for the final exchanges of the encounter, Sundowns were unable to find the back of the net and as a result suffered their ninth loss of the season.