How do I estimate ΔG⁰f for Fe2+?
This is not possible. The potentials are given only as references to the standard reduction potential for hydrogen. As such, they are not absolute.

You can convert between potential and free energy for a full redox reaction.

Actually, your statement has a deeper conceptual flaw. The standard formation free energy of the Fe(II) ion does not make any sense. The standard free energy of formation requires making something from its constituent elements in their standard states, but you cannot make an ion from its constituent neutral atoms. You'll need to account for the energy of the electrons and their binding energy to the ion. Therefore, the cation alone cannot have a standard free energy of formation.