Gas ionization sensors (GISs) based on n-type and p-type ZnO nanowires (NWs) have been fabricated and successfully tested in field ionization (FI) and field emission (FE) modes in sub-Torr atmospheres of various gases (Ar, He, N2, and O2). Novel GISs demonstrated good stability and low breakdown voltages. Significantly decreased electrical breakdowns in gases were attained in the vicinity of positively charged p-type ZnO NWs’ tips. The applied voltages were less than 40 V. These experimental outcomes can be ascribed to the fact that Ag-doped ZnO NWs have an increased number of empty states below the Fermi level and because of the observed high curvature on the NWs’ apexes. The calculated enhancement factors were 45 and 1.49 × 104 in the GIS based on n-type ZnO NWs and in the GIS based on p-type ZnO NWs, respectively. The novel GIS based on p-type ZnO NWs described here is inexpensive, simple in operation, consumes little power, and can be used for environmental monitoring and gas detection in industrial and living environments or for space applications.