Quasi-two-dimensional molecular conductor λ-(BETS)2GaCl4 shows superconductivity (SC) below 5.5 K, neighboring the dimer-type Mott insulating phase. To elucidate the origin of SC and its gap function, we carry out first-principles band calculation and derive a four-band tight-binding model from the maximally localized Wannier orbitals. Considering the spin-fluctuation-mediated mechanism by adding the Hubbard U-term to the model, we analyze the SC gap function by applying the random phase approximation. We show that the SC gap changes its sign four times along the Fermi surface (FS) in the unfolded Brillouin zone, suggestive of a d-wave-like SC gap, which only has two-fold symmetry because of the low symmetry of the crystal structure. By decomposing the SC gap into the pairing functions along the crystal axes, we compare the result to similar analyses of the well-studied κ-type molecular conductors and to the experiments.