Good reading about 64-bit .Net and the GAC... Via, Junfeng.
Here are a couple of really worthwhile entries written by Junfeng on the GAC, why you might compile platform specifc and 64-bit... I don't know if I need to bother with my intended GAC entry anymore...
https://blogs.msdn.com/junfeng/archive/2004/08/11/212555.aspx
https://blogs.msdn.com/junfeng/archive/2004/09/12/228635.aspx