  We present an entirely new sample of 388 low-mass galaxies ($M_\star \leq
10^{10} M_\odot$) that have spectroscopic signatures indicating the presence of
massive black holes (BHs) in the form of active galactic nuclei (AGNs) or tidal
disruption events (TDEs). Of these, 70 have stellar masses in the dwarf galaxy
regime with $10^8 \lesssim M_\star/M_\odot \lesssim 10^{9.5}$. We identify the
active galaxies by analyzing optical spectra of a parent sample of $\sim$23,000
low-mass emission-line galaxies in the Galaxy and Mass Assembly (GAMA) Survey
Data Release 4, and employing four different diagnostics based on narrow
emission line ratios and the detection of high-ionization coronal lines. We
find that 47 of the 388 low-mass active galaxies exhibit broad H$\alpha$ in
their spectra, corresponding to virial BH masses in the range $M_{\rm BH} \sim
10^{5.0-7.7} M_\odot$ with a median BH mass of $\langle M_{\rm BH}\rangle \sim
10^{6.2} M_\odot$. Our sample extends to higher redshifts ($z \le 0.3; \langle
z \rangle=0.13$) than previous samples of AGNs in low-mass/dwarf galaxies based
on Sloan Digital Sky Survey spectroscopy, which can be attributed to the
spectroscopic limit of GAMA being $\sim 2$ magnitudes deeper. Moreover, our
multi-diagnostic approach has revealed low-mass active galaxies spanning a wide
range of properties, from blue star-forming dwarfs to luminous "miniquasars"
powered by low-mass BHs. As such, this work has implications for BH seeding and
AGN feedback at low masses.
