$F_2$ is gas but $I_2$ is solid, because

Larger London forces are present in $I_2$ when compared to $F_2$ .The molecules of each substance attract each other through dispersion (London) intermolecular forces.Whether a substance is a solid, liquid, or gas depends on the balance between the kinetic energies of the molecules and their intermolecular attractions. Molecules at the same temp have the same average kinetic energy

In fluorine, the electrons are tightly held to the nuclei. The electrons have little chance to wander to one side of the molecule, so the London dispersion forces are relatively weak

In iodine, the electrons are so far from the nuclei that the electron clouds can easily distort. The London dispersion forces are strong.