Abstract: The KM3NeT Collaboration has tackled a common challenge faced by the astroparticle physics community, namely adapting the experiment-specific simulation software to work with the CORSIKA air shower simulation output. The proposed solution is an extension of the open source code gSeaGen, which allows the transport of muons generated by CORSIKA to a detector of any size at an arbitrary depth. The gSeaGen code was not only extended in terms of functionality but also underwent a thorough redesign of the muon propagation routine, resulting in a more accurate and efficient simulation. This paper presents the capabilities of the new gSeaGen code as well as prospects for further developments. Program summary Program title: gSeaGen CPC Library link to program files: https://doi.org/10.17632/ymgxvy2br4.2 Developer's respository link: git.km3net.de/opensource/gseagen Licensing provisions: BSD 3-Clause Programming language: C++ Nature of problem: Integration of the state-of-the-art extensive air shower Monte Carlo event generator CORSIKA [1] into the atmospheric muon simulation for water Cherenkov neutrino telescopes. The primary use case considered is the KM3NeT experiment [2], however, the code should be able to cover other similar experiments as well. The challenges in this work included interfacing the CORSIKA binary output, efficient handling of already generated events to reduce the overall computational cost, and preserving all the additional available information, which can be invaluable in physics analyses. Solution method: The readout of CORSIKA simulation was adapted from the base script provided together with CORSIKA and implemented as a standalone flux driver in gSeaGen. The propagation routine has been redesigned to support the geometry of extensive air shower simulations and to improve its efficiency in propagating particles to the detector. To ensure a reliable modelling of muon energy loss and scattering, PROPOSAL [3] was set as the default internal code for muon transport. PROPOSAL is an open-source software developed and maintained by the IceCube collaboration [4] and is a well-established solution used by the neutrino physics community. Additional comments including restrictions and unusual features: The code was tested with GENIE [5] version 3.4.0 and PROPOSAL 6.1.5. Currently, linking of gSeaGen to GENIE is mandatory, even in the case of a muon-only simulation using CORSIKA.