Using FEP in implicit solvent

GROMACS version: 2018.6
GROMACS modification: No
Hello, I am trying to do free energy perturbation (FEP) calculations in implicit solvent (GB), but this throws the fatal error “Free energy and GB not implemented.” in gmx mdrun. Is it not possible to use the implicit solvent model on free energy calculations for any version of gromacs?

My motivation for these calculations is to obtain a reference conformational entropy change for a point mutation. For this I need to run the same FEP simulations at several temperatures, and use the slope of free energy changes against temperature to evaluate the entropy.

It is not, and implicit solvent models have been removed from more recent versions of GROMACS due to incompatibilities and other reasons.