Why fluorine and chlorine are gases

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
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
As we move from fluorine to iodine, the electrons are further from the nuclei so the electron clouds can more easily distort. The London dispersion forces become progressively stronger.
At a low enough temperature, the molecules will all be solids. At a high enough temperature, they will all be gases.
It is only at a temperature  between −7^oC and 59^o C that fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.