Since taking over from Colombian coach Juan Carlos Osorio in November, Gamal had led Zamalek to five consecutive victories across all competitions.
However, Masry winger Ghiles Guenaoui ended their winning streak by scoring the winning goal just before halftime.
Following this defeat, Zamalek currently sit in 11th place in the league table with 11 points, trailing leaders Pyramids FC by six points.
Pyramids FC themselves were held to a 2-2 draw by newly-promoted side Baladeyet El-Mahalla on the same day.
On the other hand, Masry climbed to second place with 15 points.
During the game, Masry had two close opportunities to take the lead against a lackluster Zamalek side that relied on possession without posing a significant threat.
Guenaoui missed a chance from close range, hitting the side net in the 22nd minute.
Five minutes later, Masry was awarded a penalty, but Zamalek goalkeeper Mohamed Awad saved Elyes Jelassi's spot-kick.
In injury time of the first half, Fakhreddine Ben Youssef cleared the path for Guenaoui, who fired the ball into the net, giving Masry the lead.
Zamalek had a chance to quickly equalize, but Seif Jaziri's shot struck the post, while Mohamed Tarek's follow-up attempt was cleared off the line by Masry keeper Mahmoud Gad.
The second half witnessed a more balanced performance from the hosts, while Zamalek struggled to create a real threat as their shy attempts were easily dealt with by Masry's defenders and goalkeeper.
The only notable opportunity for Zamalek came in the 67th minute when substitute winger Ibrahima Ndiaye unleashed a powerful shot from inside the area, only to be denied by the crossbar.
Despite the introduction of second-half substitutes Shikaballa, Nasser Mansi, and Ibrahima Ndiaye, Zamalek failed to make the desired impact, leading to their third loss in eight league games.
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