Motivated by the recent breakthroughs in GW astronomy and in radio imaging of BHs, the last few years have witnessed a growing interest in exploring the phenomenology of BHs, compact objects and horizonless micro-states which emerge in the 'fuzzball' proposal in string theory. Semi-analytical, fully numerical techniques and, more recently, exact methods from gauge theory and 2d CFT have been developed by different communities to study the linear response to perturbations of BHs and compact objects, their multipolar structure, tidal deformation properties and absorption coefficients that are crucial to model the GW signal expected from the merger of these objects and discriminate signatures of new physics at the horizon scale. The goal of this workshop is to bring together scientists from different communities, backgrounds and expertise, to stimulate interdisciplinary interactions between ideas and techniques as diverse as gauge theories, 2d CFT, BH and GW phenomenology, string theory, and advanced numerical techniques. * A Covid Green Pass is required to access the Institute. * Due to the pandemic, participation to GGI events (in presence) is currently restricted to at most 22 people at a time. Hopefully, this constraint will be weakened in the coming months but for the present time we must comply with this rule.