Transients can come from a wide variety of different astrophysical systems. Some of the most interesting things that Gaia will see are described here:
At the end of their lives, some stars will explode and produce a bright transient which astronomers call a supernova.
What are cataclysmic variables and how does Gaia see them?
Black Hole Transients
Supermassive black holes lurking at the centres of galaxies can give rise to phenomena such as Active Galactic Nuclei and Tidal Disruption Events.
Even small stars like our own Sun can experience outbursts and flares.
Astronomy is the oldest science, and transients have been observed for longer than you might think.
Transients in the Future
What is being planned now for after Gaia?