Available grid sizes is stored in a list in the config. The conf node is called editor/grids.
The easiest way to change it is probably to creating an user config file that contains a list of your preferred grid sizes in an overwrite subtree (so it will override any stock config on grid sizes). Example: copy this config file to ~/.pcb-rnd/ and start pcb-rnd.
Start pcb-rnd and open the preferences dialog (File menu, preferences), then
If it doesn't exist, it will create the user config file (as ~/pcb-rnd/pcb-conf.lht) with the grids entered. If the config file already exists, it is overwritten (but other settings are kept) so the same method can be used to edit the grid list any time.
With the "user" role this will have effect on anything done with the same user. However, it is possible to narrow the scope and use a different role in step 3, e.g. to set up a project-specific grid (affecting only one project) or even a grid config that affects only a single board file (and saved in that board file). For more info, refer to the role specification of the conf system.