Siam orange (Citrus nobilis) is a key horticultural commodity in Indonesia, yet significant postharvest losses (20-50%) due to rapid deterioration pose a major economic and sustainability challenge. Conventional preservation methods often rely on synthetic chemicals with adverse environmental impacts. This study investigated the integrated application of the natural plant growth regulator Gibberellic Acid (GA₃) and harvest timing as an eco-friendly strategy to prolong shelf life and maintain quality, aligned with ecological engineering principles. A field experiment employed a Nested Factorial Randomized Block Design with four GA₃ concentrations (0, 25, 50, 75 ppm) and three fruit maturity stages (immature/green, physiologically mature/greenish-yellow, over-mature/yellowish-green). Results demonstrated that pre-harvest GA₃ application, particularly at 50 ppm, combined with harvesting at the physiologically mature stage, acted synergistically to enhance yield, fruit size, and nutritional quality (Vitamin C). Most notably, this combination significantly extended shelf life by approximately 18% (from 16.33 to 19.44 days at ambient storage) and drastically reduced postharvest losses, lowering fungal decay incidence from 42.72% to 15.38% and non-edible matter from 54.78% to around 17-22%. The findings establish that integrating 50 ppm GA₃ with harvest at physiological maturity offers a sustainable, practical protocol. This approach reduces dependency on synthetic postharvest chemicals, minimizes food waste, enhances resource-use efficiency, and contributes to a more resilient and eco-friendly citrus supply chain.