It is possible to save a modified item (or a newly generated item) back to your data file during Play mode.

First update your item in the master data store by calling UpdateItem with the item you’d like updated.

GDEItemData data = new GDEItemData(GDEItemKeys.Item_1);
data.some_field = "A new value";
  
GDEDataManager.UpdateItem(data);

When you are done updating items, save your updates to the data file by calling SaveToDisk.

GDEDataManager.SaveToDisk();

Note: Play mode saving is not availble when Web platform is set as the build target.