Combining multiple gerbers is called a “panel”.
Here is an example from the Purple Factory:
The tradional way to make such panels is not with a PCB program,but by post processing the Gerber files with some other program. Note that in the picture above, all the different PCB’s are still connected via small bridges.
In the last 10 to 20 years or so, it is apparently more common for PCB programs to have build in features to help with making panels. KiCad however does not have any built in support for creating panels yet. Panels can be made with KiCad, but it is a bit of a hodgepodge of creativity and possibly some helper scripts.
Here is another thread about this topic from just 2 days ago, and it also has some hints and links: