Assuming:

1.  There is a group on the system named group1
2.  There is a user on the system named user1
3.  User1 is a member of group1, but group1 is NOT user1's default group.

Do the following:

1.  Create a directory named dir.
2.  Set dir's group owner to group1.
3.  Set the sgid bit on dir.
4.  Open the directory in Nautilus.
5.  Use nautilus' template feature to create a new document in the folder.
6.  Notice that the group owner of the document is not group1.

Files created in dir by applications such as AbiWord are group owned by group1.