This study presents the integration of Additive Manufacturing (AM) with cellular metamaterials in composite sandwich structures. Experimental and numerical investigations were conducted on gyroid-based triply periodic minimal surface (TPMS) cores combined with glass fiber-reinforced polymer (GFRP) face sheets to maximize mechanical performance. Findings reveal that integrating 3D-printed cores into sandwich composites results in a lightweight structure with high load-bearing capacity. The sandwich beam system achieves seven times the load-bearing capacity and four times the stiffness of the core-only specimen. The additive manufacturing of large-scale composite metamaterials offers significant potential due to their superior mechanical properties compared to traditional two-dimensional structures. However, further research is needed to investigate the effects of printing parameters on the final part.