GROMACS modification: Yes/No
Here post your question Is it possible to run MC in gromacs?
No, it is not.
As a point of clarification, grand canonical monte carlo is feasible with GROMACS (see: https://onlinelibrary.wiley.com/doi/full/10.1002/jcc.22947 ) but requires an external tool and was designed for an older version.
If you are looking for Monte-Carlo for proteins CAMPARI ( http://pappulab.wustl.edu/campari/doc/index.html ) or MCPro/BOSS (from Jorgensen’s lab) are good places to start.