In Outlook 2003/2007 follow the following steps to export your GAL, which can then be sent to POP3/IMAP4 users.

In Outlook 2003/2007 click on File, choose New and then click on Outlook Data File.

Click Ok

PLEASE NOTE: If you will need
this file to be used by older clients then you should choose Outlook
97-2002 Personal Folders File (.pst) before clicking OK.

The default file name will be Personal Folders.pst and the default directory that is get stored in is C:\Document and Settings\\Local Settings\Application Data\Microsoft\Outlook
which is a hidden directory. We suggest naming the file to something
unique and saving it to a more accessible location such as My Documents
or Desktop.

In the Name field type something descriptive so you know that this
Personal Folder is for your GAL export. The other settings are personal
settings that you may choose to change or keep as the default. Once
done with the Name and the optional settings click on OK:

Now in Outlook switch to Folder View. This is done by click on the little folder icon in the lower left corner as seen here:

Now Right Click the new folder you just created and choose New Folder:

Type in a descriptive Name, then drop the Folder contains option down and choose Contact Items:

Now in Outlook click on Tools then Address Book or hit CTRL+SHIFT+B.
Once the Address Book is open click on Tools in the Address Book and
then Options. In the setting for Keep personal addresses in: change it
to the Folder that you just created and then click OK:

Now click on the first contact in your Global Address List and
scroll down to the end of the list. While holding the shift key and
click the last contact in the list. (This should highlight all of your
Global Address List contacts.) Right click any of the highlighted
contacts and choose Add to Contacts.

You can now click on Tools and Options again in the Address Book and
change the Keep personal addresses in: back to Contacts. After changing
that back and clicking OK you can close the address book and view your
Contacts folder to verify that the GAL has been exported to a PST.

You can now send that PST to your users that use POP3\IMAP4 and they
can import them into their contacts to have a GAL book as well.