vSPD is generally formulated as a linear programming (LP) model but under certain conditions it might be a mixed integer programming (MIP) problem. An LP/MIP solver is then clearly required. To state the obvious, the first consideration is that the solver to choose must be available with GAMS, as vSPD and GEM are GAMS-based models. There are many solvers to choose from and the Authority doesn't really endorse any particular one. That said, we have in the past trialled Gurobi, Cplex and Xpress - and all three of those solvers worked well with both vSPD and GEM. While we have never trialled Mosek, we understand that it too is an adequate solver.
GEM is frequently configured to be a very large MIP problem. A solver that performs well on MIPs should be selected if GEM is going to be the predominant model to be solved.
A test license with a limited life can be arranged if you want to try several solvers before making a decision.