  With the goal of a thorough investigation of the ionised gas and its origin
in early-type group-dominant galaxies, we present archival MUSE data for 18
galaxies from the Complete Local-Volume Groups Sample (CLoGS). This data
allowed us to study the spatially-resolved warm gas properties, including the
morphology of the ionised gas, EW(H$\alpha$) and kinematics as well as the
gas-phase metallicity (12 + log(O/H)) of these systems. In order to distinguish
between different ionisation mechanisms, we used the emission-line ratios [O
III]/H$\beta$ and [N II]/H$\alpha$ in the BPT diagrams and EW(H$\alpha$). We
find that the ionisation sources in our sample have variable impacts at
different radii, central regions are more influenced by low-luminosity AGN,
while extended regions of LINER-like emission are ionised by other mechanisms
with pAGBs photoionisation likely contributing significantly. We classified our
sample into three H$\alpha$+[N II] emission morphology types. We calculate the
gas-phase metallicity assuming several methods and ionisation sources. In
general, 12 + log(O/H) decreases with radius from the centre for all galaxies,
independently of nebular morphology type, indicating a metallicity gradient in
the abundance profiles. Interestingly, the more extended filamentary structures
and all extranuclear star-forming regions present shallow metallicity
gradients. Within the uncertainties these extended structures can be considered
chemically homogeneous. We suggest that group-dominant galaxies in our sample
likely acquired their cold gas in the past as a consequence of one or more
mechanisms, e.g. gas-clouds or satellite mergers/accretion and/or cooling flows
that contribute to the growth of the ionised gas structures.
