We report ac susceptibility and continuous wave and pulsed EPR experiments performed on ${\mathrm{GdW}}_{10}$ and ${\mathrm{GdW}}_{30}$ polyoxometalate clusters, in which a ${\mathrm{Gd}}^{3+}$ ion is coordinated to different polyoxometalate moieties. Despite the isotropic character of gadolinium as a free ion, these molecules show slow magnetic relaxation at very low temperatures, characteristic of single molecule magnets. For $T\lesssim 200\mathrm{mK}$, the spin-lattice relaxation becomes dominated by pure quantum tunneling events, with rates that agree quantitatively with those predicted by the Prokof’ev and Stamp model [ Phys. Rev. Lett. 80, 5794 (1998)]. The sign of the magnetic anisotropy, the energy level splittings, and the tunneling rates strongly depend on the molecular structure. We argue that ${\mathrm{GdW}}_{30}$ molecules are also promising spin qubits with a coherence figure of merit $Q_{\mathrm{M}}\gtrsim 50$.