I have seen many papers on folding and unfolding of peptides using Replica exchange with solute tempering (REST). How do we implement this method in GROMACS > 5.1?
You might want to refer to the two papers linked below.
The first one tells you how to scale your force field parameters by specific factors; basically, you'll have to make local copies of your force field files and write a script to scale the parameters for protein atom types.
The second presents HREX, a patched version of Gromacs (AFAIK limited to 4.6, so it might not be possible to run REST in gmx > 5.1) that allows to perform replica exchange with arbitrary energy functions; from what I recall, the problem is that standard Gromacs has replex limited to the lambda code, so this workaround was necessary.
I assume you're fluent with Gromacs and scripting - setting up REST might not be as straightforward as, say, running standard REMD.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3172817/
https://arxiv.org/pdf/1307.5144.pdf
