Most early-type galaxies are not devoid of cold and warm gas. The origin and ionization of this gas reveal the intriguing ongoing evolution of these galaxies. In most cases, the warm ionized gas shows emission-line spectra similar to low-ionization nuclear emission-line regions (LINERs). Their ionization mechanism has been hotly debated. We will present evidence from line ratio gradient that rules out AGN and shocks as the dominant ionization mechanism, and suggests the ionizing sources follow the stellar density profile. Hot evolved stars are the favorite candidates but bring new puzzles. This finding allows us to obtain a gas-phase metallicity calibration in these early-type galaxies, using the line emission. We will show how the metallicity of the warm gas depends on stellar mass and stellar age, and what it tells us about the origin of the warm gas in these galaxies.