A recent result by J. Šaroch and J. Šťovíček asserts that there is a unique abelian model structure on the category of left R-modules, for any associative ring R with identity, whose (trivially) cofibrant and (trivially) fibrant objects are given by the classes of Gorenstein flat (resp., flat) and cotorsion (resp., Gorenstein cotorsion) modules. In this paper, we generalise this result to a certain relativisation of Gorenstein flat modules, which we call Gorenstein B-flat modules, where B is a class of right R-modules. Using some of the techniques considered by Šaroch and Šťovíček, plus some other arguments coming from model theory, we determine some conditions for B so that the class of Gorenstein B-flat modules is closed under extensions. This will allow us to show approximation properties concerning these modules, and also to obtain a relative version of the model structure described before. Moreover, we also present and prove our results in the category of complexes of left R-modules, study other model structures on complexes constructed from relative Gorenstein flat modules, and compare these models via computing their homotopy categories.