This work investigates the integration of Additive Manufacturing (AM) techniques with cellular metamaterials integrated into composite sandwich beam systems. The study proposes an approach that combines composite materials for the face sheets with cellular structures using a Triply Periodic Minimal Surface (TPMS) Gyroid structure for the core to achieve maximum lightweight and load-bearing capabilities. The experimental and numerical campaigns were utilized for the material testing of 3D printing polymeric material reinforced with chopped carbon fibre (CF). To validate the composite sandwich structure, three bending experiments were conducted: (a) bending of the "core only" was performed to calibrate the material for the given print parameters; (b) bending of the "sandwich beam" composite with a periodic and homogenous Gyroid core bonded with glass fibre reinforced polymer (GFRP) face sheets; (c) the "arch beam" composite with the change in outer crosssection dimension with the same periodic and homogenous Gyroid core. The FEM analysis was combined with Digital Image Correlation (DIC) results to determine the bending stiffness of the sandwich beams and to detect the failure modes. It was discovered that integrating 3D printing into load-bearing structures through the composite "sandwich beam" system resulted in seven times increase in load-bearing capacity and four times increase in stiffness compared to results obtained with the "core only" structure.