From the Git index-format.txt file, regarding the mode:

32-bit mode, split into (high to low bits)

    4-bit object type
      valid values in binary are 1000 (regular file), 1010 (symbolic link)
      and 1110 (gitlink)

    3-bit unused

    9-bit unix permission. Only 0755 and 0644 are valid for regular files.
    Symbolic links and gitlinks have value 0 in this field.

Also, a directory object type (binary 0100) and group-writeable (0664 permissions) regular file are allowed as indicated by the fsck.c fsck_tree method. The regular non-executable group-writeable file is a non-standard mode that was supported in earlier versions of Git.

This makes valid modes (as binary and octal):

  • 0100000000000000 (040000): Directory
  • 1000000110100100 (100644): Regular non-executable file
  • 1000000110110100 (100664): Regular non-executable group-writeable file
  • 1000000111101101 (100755): Regular executable file
  • 1010000000000000 (120000): Symbolic link
  • 1110000000000000 (160000): Gitlink