The easiest would probably be to download a default/example mdp file (from some tutorial or so on), and then modify it per your needs.

you can just go to the gromacs tutorial website and there you'll find the .mdp file (written as "here") at every step when required. As mentioned in the answer above you may then modify any of the parameters as per your study.

You can get the .mdp files for free energy simulations in the umbrella sampling method here:

https://group.miletic.net/en/tutorials/gromacs/5-umbrella/#analysis