The recent discovery of a neutrino emitted by a blazar, an active galactic nucleus known to emit photons across the entire electromagnetic spectrum up to the highest energies, has been one of the key events confirming that we are witnessing a multi-messenger era in astrophysics. Gamma-ray astronomy is an important part of this picture, being the discipline that studies photons at the highest energies, produced in the most violent processes in the Universe. With our proposal we aim to consolidate the gamma-ray astronomy research conducted in international collaborations as MAGIC (Major Atmospheric Gamma-ray Cherenkov telescopes) and CTA (Cherenkov Telescope Array), and moreover to widen the Croatian-based contribution to this field moving towards the theoretical interpretation of the results in a multi-messenger context. This means that, on top of the proven experience coming from years of scientific results in the field of gamma-ray astronomy, the team will also work on the theoretical interpretation and modelling of the results in a broadband view and on the investigation of quantum gravity effects such as Lorentz Invariance Violation. This expansion of horizons is timely since the understanding of the theoretical models is the key to unveil the emission process of different extragalactic and galactic sources, including the newest ones discovered at the highest energies, the so-called PeVatron sources. To explore such new energy frontiers we will also work in the development of a novel Cherenkov detector for the new strategic collaboration with the SWGO (Southern-Wide Gamma-Ray Observatory).