We have investigated the transport properties, resistivity and Hall effect, in a series of underdoped GdBa2Cu3Ox thin films grown by off-axis magnetron sputtering. We find a systematic correlation between the critical temperature T-c and the inverse Hall constant R-H(-1), related, in simple models, to the carrier concentration n. Our experimental thin film T-c(n) data are in good agreement with the temperature-doping phase diagram obtained for YBa2Cu3Ox, single crystals. By measuring the activation energies in the liquid vortex phase, and by using a 2-dimensional model for vortex dynamics, we have extracted the penetration depth of these samples, and studied the relation between the carrier concentration and the superfluid density to probe the role of phase fluctuations on superconductivity.