Software recommendation for protein in electric field modeling?
MD
Broadly this could be attempted with Molecular Dynamics simulations. As you discovered in your search, one of the most popular softwares is free: GROMACS. This is an atomistic approach to the problem. It also has many electrostatic parameters.
I would point out that generally MD requires careful and robust methods and often an expert lending hand if the results are to be trusted. In order to do MD at the timescales for observing rotating molecules in the field, massive amount of HPC computational time will be required. This is a full blown project to get it to an appropriate level for publication.
This is computationally intensive.
Quick Electrostatics
If a HPC is not available to you then MD may not be an option. If all you want to do is visualise the location of the charge and make qualitative predictions or visualisations, PyMol and ABPS might be a quicker combo. It is also good for quantitative analysis, but obviously is a static model.