The acoustic absorption of a porous structure within a specific frequency range can be tuned by varying its porosity along its thickness. In this work, triply periodic minimal surfaces (TPMS) are employed to generate graded porous structures, where the continuous porosity gradient is controlled by a mathematical function involving geometric parameters. A hybrid homogenization technique, combined with the transfer matrix method (TMM), is used to predict the normal incidence absorption coefficient of the graded TPMS structure. The porosity distribution along the thickness is then optimized using a global search method combined with a local gradient-based solver to maximize acoustic absorption within a target frequency range. The optimization results suggest that a combination of high- and low-porosity layers achieves broadband impedance matching conditions by shifting the so-called quarter-wavelength resonance frequencies. The design of the TPMS absorbers is validated through impedance tube measurements of 3D-printed samples.