At the end of the twentieth century, Á. Császár pointed out the possibility of the transition from topology to generalized topology and vice versa. One of the tools used for this purpose are semi-open sets. In this paper, we investigate properties of functions in GTS in the context of semi-open sets, and problems connected with transfer of properties of functions defined and assuming their values in primal spaces to functions considered as mappings in the spaces generated by primal spaces.