As stated in the Python scripting API documentation, I can access the “Grid Spacing” as GSFont.grid and the “Subdivision” as GSFont.gridSubDivisions. Also, GSFont.gridLength returns the value GSFont.grid / GSFont.gridSubDivisions. There is no problem:
In the .glyphs file, however, GSFont.grid is stored as gridLength instead of grid:
I believe this is a historical naming inconsistency caused by an early renaming. The Subdivision was added later, that’s why there was suddenly a need to differentiate between grid step and grid length, which were the same thing before.