Fat reduction in meat products often compromises texture, water retention, and sensory quality. Emulsion gels have been proposed as effective fat replacers, but their properties depend on the preparation method. This study aimed to evaluate the functional properties of Gracilaria fisheri agar-based emulsion gels prepared using cold-set and hot-set methods, and to determine their suitability as fat replacers in fish sausages. The two emulsion gels were first compared for gel strength, water-holding capacity (WHC), and viscoelastic behavior. The hot-set emulsion gel (HEG) showed slightly higher gel strength (994.95 ± 62.20 g) than the cold-set emulsion gel (CEG) (942.42 ± 57.54 g), with no significant difference. In contrast, CEG exhibited significantly (p < 0.05) higher water-holding capacity (83.61 ± 0.43%) compared with HEG (68.78 ± 2.52%). Both gels showed similar viscoelastic behavior and crossover temperature (~80 °C). Given its superior water retention, CEG was incorporated into fish sausages at different fat replacement levels (0–100%). During storage (7 days, 4 °C), lightness (L*), yellowness (b*), pH, and oxidative stability of raw sausages changed significantly (p < 0.05), although thiobarbituric acid (TBA) values (0.04–0.87 mg MDA/kg) did not differ from the control. For cooked sausages, 50% fat replacement with CEG produced samples with comparable physicochemical and textural properties (adhesiveness, springiness, resilience, cooking loss <10%) to the control. Importantly, oxidative stability improved significantly (TBA: 0.03 mg MDA/kg). Overall, G. fisheri agar-based cold-set emulsion gels, particularly at 50% fat replacement, represent an effective fat substitute that maintains product quality while enhancing oxidative stability. These findings highlight the potential of seaweed-derived hydrocolloids for developing healthier meat products aligned with dietary recommendations for reducing fat intake.