The GNOME desktop used to use the package
gnome-screensaver for screensavers.
The thing is, gnome-screensaver became not just responsible for screensavers, but also for the system lock screen.
In a possibly surprising twist of fate, it has now removed the screensavers! So despite it still being called gnome-screensaver, all it does now is suspect/wake the screen and act as a lock screen.
Why remove screensavers?
They work contrary to modern power saving, where the approach is to put the screen into sleep mode and let the CPU/GPU idle. A screensaver keeps the screen on and keep the CPU and/or GPU doing a small amount of work. This will use more power, generate more heat, etc. This is particular important on battery powered devices like laptops and tablets.
They don't really do all that much to save screens anymore. Modern screens aren't really as susceptible to screen burn-in as older screens. A lot of us now use screens at work for 8+ hours a day with taskbars, etc constantly visible, and yet it doesn't really cause much noticeable burn-in.
That said, some people still want screensavers for the same reason they've always wanted them - they look pretty (and they use a desktop or something where they don't care about energy use).
xscreensaver package still contains screensavers, though with some differences. If you also install
xscreensaver-gl-extra then you'll get even more screensaver options. Once installed you can configure screensavers from the GUI (search for "screensaver").
Note however: this will effectively disable gnome-screensaver, so you will no longer see gnome-screensaver's lock screen but xscreensaver's lock screen, which will look and behave differently.