To change the GAL in Exchange 2010 to display your users last name before there first it is a two step process.
The first step is you need to use ADModify.net to change all of your existing users in bulk
Admodify can be downloaded here – http://admodify.codeplex.com/releases/view/6065
The second step is you need to use ADSIEDIT to change it for all of your users moving forward.