Multipartite Einstein-Podolsky-Rosen steering is an essential resource for quantum communication networks where the reliability of the equipment at all of the nodes cannot be fully trusted. Here, we present the experimental generation of a highly versatile and flexible repository of multipartite steering using an optical frequency comb and ultrafast pulse shaping. Simply modulating the optical spectral resolution of the detection system using the pulse shaper, this scheme is able to produce on-demand four-, eight-, and 16-mode Gaussian steering without changing the photonics architecture. We find that the steerability increases with a higher spectral resolution. For the 16-mode state, we identify as many as 65 534 possible steering bipartitions existing in this intrinsic multimode quantum resource, and demonstrate that the prepared state steerability is robust to mode losses. Moreover, we verify four types of monogamy relations of Gaussian steering and demonstrate the constraint imposed by one of them to be strongly lifted. Our method offers a powerful foundation for constructing quantum networks in real-world scenarios. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.