The geometry and vibrational frequencies of the GdX3 (X = F and Cl) molecules have been analyzed at the ab initio level with extended basis sets, employing relativistic effective core potential, and evaluating electronic correlation by means of second-order perturbative (MP2) and coupled cluster (CCSD and CCSD(T)) methods. Anharmonicities, temperature, and inert-gas matrix effects have been explicitly included. The MP2, CCSD, and CCSD(T) calculations on the systems in the gas phase indicate a trigonal planar equilibrium structure for GdCl3 and a quasiplanar geometry for GdF3. Vibrationally averaged bond angle, evaluated by means of a simple one-dimensional treatment, is considerably smaller than the equilibrium value and both molecules have a pyramidal thermal average structure, in agreement with recent electron diffraction measurements. The theoretical estimate of Gd-X bond lengths depends on both the electronic correlation treatment and the basis set quality, thus indicating the desirability of high-level calculations. Experimental and theoretical comparison becomes quantitative after including thermal correction. Anharmonic vibrational frequencies have been computed through the vibrational self-consistent field method followed by the second-order perturbation correction. For both gadolinium trihalides, the nu(2) out-of-plane bending potential shows a huge "negative" anharmonic form and hot bands fall at considerably higher energies than the fundamental one. Although the anharmonicities for the remaining modes are small, they are important for a correct interpretation of experimental IR spectra. The inert-gas matrix interactions, modeled by coordination of one and two inert-gas molecules GdX(3)(.)IG(m) (IG = Ne, Ar, Xe, and N-2; m = 1 and 2), are substantial and GdX3 structures strongly depend on the number of coordinating molecules and on the interaction strength. As a consequence, all normal-mode frequencies slightly diminish as GdX3--IG(m) interactions grow, while the nu(2) out-of-plane bending frequency significantly increases.