Gallium has a slightly higher first ionisation enthalpy ($579\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) than aluminium ($577\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$), contrary to the usual group trend. This is mainly because

1. Athe ten $3d$ electrons of $\mathrm{Ga}$ shield the nuclear charge poorly, so the effective nuclear charge on the $4p$ electron is higher than expected
2. B$\mathrm{Al}$ has a $3d$ subshell that is completely filled
3. C$\mathrm{Ga}$ has a half-filled $4p$ subshell
4. D$\mathrm{Ga}$ has a smaller number of shells than $\mathrm{Al}$