The uniqueindentifier datatype is a special type which allows you to identify a row by using a (globally) unique GUID.
You can read this article for more details: http://www.sqlteam.com/article/uniqueidentifier-vs-identity
Normally, you will configure the MsSQL Column as :
 | RowGuid = Yes |
| Default Value (or binding) = Newguid() |
If this is your case it means that it should be treated as an auto-numbered column.
GenWise can't determine this automatically so you will need to manually set the AutoNumber=Server as shown in image below:
XML Mapping (special generator)
When setting correctly the autonumber property for the column you will notice that the XML mapping will change to use a special NHibernate ID generator : guid.
<class name="Contract"
table="[dbo].[Contracts]">
<id name="ContractID"
access="nosetter.camelcase-underscore"
column="`ContractID`"
type="System.Guid" >
<generator class="guid" />
</id>
Example Unit Test
to prove the behaviour here is a small unit test that inserts a new object without assigning the PK, the generator (guid) will set a value after the insert is done.
[Test,Explicit]
public void InsertTest()
{
ClubFactory factory = new ClubFactory(SessionScope.PerOperation);
int countriesCount = factory.GetAll().Count;
Assert.Greater(countriesCount, 1);
Club club = new Club();
club.Description = "Sebastian";
// Test that the default value before the insert is null.
Assert.AreEqual(club.ClubID , Guid.Empty);
factory.Insert(club);
// After the insert the GUID get;s filled with the generator guid.
Assert.AreNotEqual(club.ClubID, Guid.Empty);
Assert.AreEqual( countriesCount+1 , factory.GetAll().Count);
}