Slow insert in SQL Server 2008
Why no clustered index? Why no primary key?

This is most likely your problem: you don't have any order to the table (in the sense of, say, an IDENTITY column) this you are inserting into a heap

See


https://stackoverflow.com/q/5094400/27535 (SO)
http://sqlblog.com/blogs/tibor_karaszi/archive/2008/08/14/are-inserts-quicker-to-heap-or-clustered-tables.aspx (Tibor Karaszi)


Edit, after update.

Having a non-unique clustered index requires a uniquifier (DBA.se links) per duplicate gn_ObjectId entry. This is not a good clustering key, which is best when it is


numeric
monotonically increasing
narrow


I also assume you haven't tried a IDENTITY column then...? Try it, report back..

Edit 2, as @JNK says, an ID column is not an overhead in this situation because of how your writes will be managed on disk