This study deals with GFRP/ABS/GFRP sandwich panels with FDM-printed lattice cores. The main aim is developing extremely economic and efficient structural panels for general applications. ABS is used for thin core beams and GFRP for the skin. Several core tested topologies (FCC, BCC, SFCC and others) are employed; compression tests confirmed that core geometry is critical for stiffness, density and failure modes: FCC showed progressive collapse, while SFCC and BCC exhibited sudden strength drop. Subsequently, Flexural tests (ASTM C393) indicated SFCC and SFBCC offer the best performance, while cores gave a balanced compromise; failures were mainly due to core ruptures leading to ABS/GFRP debonding. Finite element models were used to optimize lattice structure dimensions, according also to printer limits and reproduce load effects, but did not match experimental test due to process variability. Critical point was determination of minimal lattice cell and beams dimension to be used, keeping commercial FDM process acceptable for required beam shapes. FDM lattice cores reinforced with glass fiber proved extremely lightweight and strong sandwich structures, combined with low costs due to economic fiber reinforcement and basic processes for core performances.