To deploy referenced project assemblies to GAC through SharePoint WSP:
1. Open the Package designer in the SharePoint project.
2. Select the Advanced tab.
3. Click the Add button.
4. Select the Add Assembly from Project Output menu item.
5. Use the Source Project dropdown to select which project in your solution should have its assembly included.
6. Make sure the Deployment Target is set to GlobalAssemblyCache.
7. Click OK, Build & Deploy the Solution, the referenced assemblies should be in GAC.