We have investigated spin and carrier dynamics of resident holes in high-mobility two-dimensional hole systems in GaAs/Al0.3Ga0.7As single-quantum wells at temperatures down to 400 mK. Time-resolved Faraday and Kerr rotation as well as time-resolved photoluminescence spectroscopy are utilized in our study. We observe long-lived hole-spin dynamics that are strongly temperature dependent, indicating that in-plane localization is crucial for hole-spin coherence. By applying a gate voltage, we are able to tune the observed hole g factor by more than 50%. Calculations of the hole g tensor as a function of the applied bias show excellent agreement with our experimental findings.