  To understand the origin of extended disks of low-surface brightness (LSB)
galaxies, we studied in detail 4 such systems with large disks seen edge-on.
Two of them are edge-on giant LSB galaxies (gLSBGs) recently identified by our
team. The edge-on orientation of these systems boosts their surface
brightnesses that provided an opportunity to characterize stellar populations
spectroscopically and yielded the first such measurements for edge-on gLSBGs.
We collected deep images of one galaxy using the 1.4-m Milankovi\'c Telescope
which we combined with the archival Subaru Hyper Suprime-Cam and DESI Legacy
Surveys data available for the three other systems, and measured the structural
parameters of the disks. We acquired deep long-slit spectra with the Russian
6-meter telescope and the 10-m Keck II telescope and estimated stellar
population properties in the high- and low-surface brightness regions as well
as the gas-phase metallicity distribution. The gas metallicity gradients are
shallow to flat in the range between 0 and -0.03 dex per exponential disk scale
length, which is consistent with the extrapolation of the gradient -- scale
length relation for smaller disk galaxies. Our estimates of stellar velocity
dispersion in the LSB disks as well as the relative thickness of the disks
indicate the dynamical overheating. Our observations favor mergers as the
essential stage in the formation scenario for massive LSB galaxies.
