We have studied the effect of AuCl4 functionalization on the conductivity of carbon nanotube networks by first-principles electronic structure calculations. The functionalization results from treating carbon nanotube networks by dissolved AuCl3. Band structures and electronic transmission functions for single-walled semiconducting carbon nanotubes with physisorbed AuCl4 anions are computed using the density functional theory. The resulting p-type doping of nanotubes accompanied by a downshift of the Fermi level and balanced by negatively charged AuCl4 anions make the nanotubes metallic. Moreover, the influence of AuCl4 functionalization on the conductance of the junction between two semiconducting carbon nanotubes is considered. Increasing the AuCl4 coverage lowers the Fermi level rapidly until it is pinned by a van Hove singularity of the nanotube electronic structure. For these doping levels our calculations based on the density-functional nonequilibrium Green's function method show a significant increase in the intratube electron transmission. Moreover, our electron transport calculations for crossed nanotubes indicate a simultaneous increase in the intertube conductance. These factors explain the experimentally observed robust conductivity improvement of carbon nanotube networks treated by dissolved AuCl3.