How to add phosphate ions in the gromacs

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GROMACS modification: Yes/No
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How to add phosphate buffer instead of NaCl or KCL?
I would like to run a simulation with Phosphate buffer to replicate exact experimental system,

Polyatomic species can be inserted with gmx insert-molecules, provided you have a coordinate file for whatever it is you need. Most experiments are conducted at such low concentration of such species that you will only get a few molecules in the simulation and their effects are irrelevant.