The construction sector is seeking sustainable alternatives to conventional building materials to reduce carbon emissions and promote circular economy practices. This study developed gypsum–biochar composites using date-palm-waste biochar at a fixed 20:80 biochar:gypsum mass ratio and evaluated water content, polymeric additives (SBR, RDP, and BT), and drying conditions. FTIR, XRD, SEM-EDS, FESEM, and BET characterized the raw materials. Optimum water contents were 55 wt% relative to gypsum and 70 wt% relative to biochar. Excess water reduced strength, whereas insufficient water impaired workability. SBR gave the best room-temperature performance among the additives. Oven drying at 60 °C for 24 h increased the strengths of SBR-, RDP-, and BT-modified composites from 3.60, 2.43, and 2.97 MPa to 6.35, 5.89, and 6.63 MPa, respectively; the unmodified composite reached 8.25 MPa. The results support the use of waste-derived biochar in lightweight, non-load-bearing interior components, although other biochar replacement levels and durability properties require evaluation.