You can force Exchange 2007 to use a specific Global Catalog server using PowerShell cmdlets.
Use the -StaticConfigDomainController -StaticDomainController -StaticGlobalCatalog string of the Set-ExchangeServer cmdlet to set it or set exclusions using: -StaticExcludedDomainControllers
Set-ExchangeServer -StaticConfigDomainController -StaticDomainController -StaticGlobalCatalog