The latest build (3.0.4 (3097)) won’t let me save file as format 1 for a reason I can’t understand:
The property “styleMapFamilyNames” in the Instance: “Italic” can’t be saved as format 1.
I can’t see what’s wrong with this entry.
Glyphs 3 can edit Glyphs 2 files, but some of the new data of a Glyphs 3 file can not be stored in a Glyphs 2 file. As the alert says, open Font Info (File → Font Info…, ⌘I), select the Other tab and change the file format version to “Version 3”.
Saving as format 1 is precisely what I need.