In "6-31+g(d)", the "g" has nothing to do with diffusion or polarization.
"d" means polarization on heavy atoms, this is absolutely necessary. In short, always add it. Sometimes you also add "p", "6-31g(d,p)" or 6-31g**
"+" means diffusion. This is normally not necessary. Only use in a system with many negative charges and non-bond interaction is what you care. In short, only use it when you know.