Multiple Masters, part 3: setting up instances | Glyphs says
Fear not, there is a simple solution. In the Custom Parameters section, at the bottom of the window, add a new parameter by clicking the plus button. Set the Property to weightClass and its Value to something a little larger or smaller, e.g., 255 or 245. Remember: the smaller the value, the higher up in the font family submenu it will end up.
I don’t recommend usig 245 as an example, because 249 or lower will be auto-bolded (smeared) in Windows