How is the structure passed to the buckingham_input method of the GulpIO class? Do you have any method to pass this structure?
https://pymatgen.org/pymatgen.command_line.gulp_caller.html?highlight=gulpio#pymatgen.command_line.gulp_caller.GulpIO
from pymatgen.command_line.gulp_caller import GulpIO
from pymatgen.core.structure import Structure
gulp = GulpIO()
structure = Structure.from_file('NaCl.gin')
gulp.buckingham_potential(structure)