  To better characterize the upper end of the galaxy stellar mass range, the
MUSE Most Massive Galaxies (M3G) Survey targeted the most massive galaxies
(M$>10^{12}$ M$_{\odot}$) found in the densest known clusters of galaxies at
$z\sim0.046$. The sample is composed by 25 early-type galaxies: 14 BCGs, of
which 3 are in the densest region of the Shapley Super Cluster (SSC), and 11
massive satellites in the SSC. In this work we aim at deriving the spatial
distribution and kinematics of the gas, and discussing its ionisation mechanism
and origin in the optical wavelength range with MUSE data. We fit the continuum
of the spectra using an extensive library of single stellar population models
and model the emission lines employing up to three Gaussian functions. In the
M3G sample, ionized-gas was detected in 5 BCGs, of which one is in the densest
region of the SSC, and 6 massive satellites in the SSC. Among these objects,
[OI] and [NI] were detected in 3 BCGs and one satellite. The gas is centrally
concentrated in almost all objects, except for 2 BCGs that show filaments and 2
massive satellites with extended emission. The emission line profiles of 3 BCGs
present red/blueshifted components. The presence of dust was revealed by
analysing Balmer line ratios obtaining a mean $E(B-V)$ of 0.2-0.3. The
emission-line diagnostic diagrams show predominately LINER line ratios with
little contamination from star formation. The gas was detected in 80% of fast
rotators and 35% of slow rotators. The orientations of stellar and gaseous
rotations are aligned with respect to each other for 60% of satellites and 25%
of BCGs. The presence of misalignments points to an external origin of the gas
for 3 BCGs and 2 satellites. On the other hand, some of these systems are
characterized by triaxial and prolate-like stellar rotation that could support
an internal origin of the gas even in case of misalignments.
