We present a gate-voltage-tunable transmon qubit (gatemon) based on planar InAs nanowires that are selectively grown on a high-resistivity silicon substrate using III-V buffer layers. We show that low-loss superconducting resonators with an internal quality of 2 x 10(5) can readily be realized using these substrates after the removal of buffer layers. We demonstrate coherent control and readout of a gatemon device with a relaxation time, T-1 asymptotic to 700 ns, and dephasing times, T-2* asymptotic to 20 ns and T-2,T-echo asymptotic to 1.3 mu s. Further, we infer a high junction transparency of 0.4-0.9 from an analysis of the qubit anharmonicity.