Installera BizTalk adapter

http://msdn.microsoft.com/en-us/library/aa561871(BTS.70).aspx

Your assemblies do not have to be in the GAC. I just looked on 2 BizTalk applications that are installed and running, and the assemblies for the adapters are not in the GAC
I inherited a VS setup project that appears to handle the registry stuff for you. I don't know if you get this out of the box when creating a new setup project and adding references to your BizTalk project.
You need to register the custom adapters using a registry file.
This file can be created using a tool that comes in the SDK and is installed on my computer here: C:\Program Files\Microsoft BizTalk Server 2006\sdk\Utilities\AdapterRegistryWizard
Documentation on the Registry File: http://msdn2.microsoft.com/en-us/library/aa560561.aspx
Documentation on the AdapterRegistryWizard: http://msdn2.microsoft.com/en-us/library/ms966446.aspx

You can install the adapter assemblies in the GAC, and then leave out the file paths in the registry file. From the notes on the registry file:
For OutboundAssemblyPath and AdapterMgmtAssemblyPath we recommend that you not include the local path in the property value, because the configuration could break when installed on different server locations. A better choice is to use a strong name and install it in the global assembly cache.