This study aims to explore the mechanical behaviour of polymeric gyroid structures under compression within the context of orthotic insoles, focussing on custom optimisation for lower peak plantar pressures. This research evaluates the compressive response of gyroid structures using a combination of experimental testing and numerical modelling. Stereolithography was used to manufacture gyroid samples for experimental tests, and explicit finite element analysis was used to model the gyroid's response numerically. Hyperfoam, first-order polynomial, and second-order polynomial hyperelastic constitutive models were considered to homogenise the mechanical response of the structure. The homogenised properties of the structure were then implemented in an optimisation algorithm to obtain the optimal gyroid structure for a given subject by minimising the standard distribution of plantar pressures. Findings indicate that the compressive response polymeric gyroid structures can be represented with a homogeneous material. The hyperfoam model was chosen due to its accuracy and interpolation quality. The optimisation process successfully identified configurations that maximise the mechanical advantages of gyroid lattices, demonstrating significant improvements in plantar pressure distributions. The optimised insole showed a 30% reduction in the standard deviation of the plantar pressure and a 10% reduction in the peak stress. The optimisation method reduced peak pressures by 12.2 kPa compared to a traditional medium-density Poron orthotic insole, and 94.3 kPa compared barefoot conditions. The mechanical response of gyroid structures has successfully been modelled, analysed and homogenised. The study concludes that custom gyroid-based orthotic insoles offer a promising solution for personalised foot care.