I found this finally on the Internet and it answers the question.
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The order in which the address lists are shown is determined by the order in
which the services (MS Exchange, Outlook Address Book, and Personal Address
Book) were added into your profile.

From this perspective, I don't think you'll be able to separate the GAL
listing from the other address lists, as they are all part of the MS
Exchange service.  Perhaps there is a way it can be set on the server side
by your IT folks, I'm not sure about that.

So you can't get exactly what you want.  But you can probably get close.
Assuming that your Exchange service was added to your profile prior to the
OAB or PAB services: if you delete the Exchange service from your profile
(make sure you record all values and settings in that service's properties
before deleting it!!!!), then add it back again, you should find your OAB
and PAB listed first in the drop-down, followed by the GAL and then all of
the other lists.  I think that should leave you a lot closer to your goal
than what you have now.

One additional note: if you do go messing around with the above order, I
recommend when you're done that you check the Addressing tab within your
profile; specifically you may need to adjust the "When sending mail" list to
be in the desired order in terms of how you want the lists resolved (for
example, you may need to move the GAL up to the top if you want it checked
first).