This paper presents the application of additively manufactured (3D-printed), shape-optimized core fillers in a full-scale composite aircraft landing gear structure. Preliminary investigations included a comparison of conventional PVC foam cores and 3D-printed honeycomb cores using three-point bending tests. The main part of the study focused on the design and integration of geometrically complex 3D-printed fillers into a composite landing gear leg. The fillers enabled accurate reproduction of the variable, curved and twisted internal geometry, integration of brake-line channels, and manufacture of the closed-section component in a single lamination and curing cycle. The manufactured landing gear was subsequently subjected to airframe-level strength tests and a separate component-level destructive test. The landing gear successfully sustained the required structural verification loads, while the maximum load recorded during the destructive test reached approximately 219% of the calculated maximum allowed load. No failure initiation or structural instability associated with the additively manufactured fillers was observed. The proposed solution was implemented in the production process of a light aircraft, demonstrating its practical applicability as a process-enabling element in the manufacture of complex composite structures.