How do I reference an assembly from the GAC in a PowerShell module?
Unless you're going to deploy the SMO assemblies as private assemblies, then I wouldn't recommend loading them from direct references.
To load from the GAC, use the Add-Type cmdlet with the fully qualified assembly name...
Add-Type -AssemblyName "Microsoft.SqlServer.Smo, Version=12.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91"Related:How do I use Add-Type to load Microsoft.Web.Deployment?
You can also specify just the assembly without the version info:
Add-Type -AssemblyName "System.Xml.Linq"