How to add force field in GULP using material studio

i am working on piezoelectric properties of BiFeO3 but force filed of Bi is missing

Force fields have to be parameterised by someone, so if there is no existing force field for your system then you’ll need to fit one. There are universal force fields (such as pGFNFF and UFF), but you should read the papers first to see if they are suitable for what you want.