Pirates advanced courtesy of Thamsanqa Gabuza's 103rd minute winner after Celtic's Kabelo Mahlasela cancelled out Abbubaker Mobara's second-half opener at Orlando Stadium.
The Buccaneers, cup holders SuperSport United, Chippa United and Golden Arrows all booked their spot in Monday night's last-four draw.
Gcaba told SuperSportTV: "We knew Celtic would come here looking for a win to advance to the next stage. We had to grind, it was another hard win for us to be in the semi-finals - we did it through teamwork."
The big defender insisted the team is doing their utmost to try and protect goalkeeper Siyabonga Mpontshane.
"We do score, we also concede, but we try not to concede as many goals as they create. We tried by all means to close them and to seal it at the back to make it easier for the keeper, not to concede any goals."
Gcaba also revealed coach Kjell Jonevret emphasised "teamwork, to be solid and to communicate at all times" during the half-time break in Soweto.