DFT is most commonly used to calculate minimized energy structures. This is likely what your project is on, unless you're doing ab initio molecular dynamics, or excited state DFT (e.g. time-dependent DFT) calculation. In this case, your calculations are equivalent to finding the lowest enthalpy ground state minus the vibrational zero point energy. Enthalpy $H$ will be equal to the total energy $E$ unless you're also applying some external stress (i.e. pressure), which would correct the enthalpy by a $PV$ term.
Since your calculations are at $T =0$ (equations of motion are not even employed), the energy (or enthalpy) minimization is also equal to the Gibbs free energy $G$.
If you want to determine which polymorph is most stable at finite temperature and pressure (lowest $G$), you need a more sophisticated calculation than straightforward DFT structure minimization. A common method is to correct the minimized energy for vibrational energy based on a harmonic or quasiharmonic vibrational calculation.