gamma-gamma collider is a challenging new type of particle collider that can produce complementary and unique new physics compared to other type of colliders (hh, ee and eh). It can be used as a Higgs factory with relatively low electron beam energy than an e+e‒ collider. For future acceleration technique, such as plasma wakefield acceleration, it is hard to accelerate e+ particles. It is therefore important to develop a technology that will allow physicists to accelerate e‒ only and still access annihilation reactions with precisely understood point-like interactions. gg collider fills this need. Furthermore, because it requires a high power laser system in addition to an accelerator, it provides a case for two different communities – accelerator and laser – to joint force and work together, which could have profound impact on the future of our field. This workshop will discuss gg colliders based on several different technologies: linear collider (e.g., ILC, CLIC), recirculating linac (e.g., SAPPHiRE, HFiTT), circular collider (e.g., FCC-ee, CEPC) as well as advanced accelerator concepts (AAC, e.g., laser driven plasma acceleration, beam driven plasma acceleration, dielectric wakefield acceleration). It will also discuss AAC-based light source as a possible near term application.