In Production (APP/WFE) server we may not have Visual Studio installed in, and we don't have gacutil.exe. In worst cases manually if you need to add the dll's to GAC for workaround you can follow the below steps.
1. Start--> Local Security Policy --> Local Policies --> Security Options --> and right click on the 'User Account Control: Run all administrators in Admin Approval Mode' and disabled it.
You should restart the Server to turn off the UAC Admin mode.
2. Add the dll's to (drag and drop) assembly(C:\windows\assembly\)
3. Once you add the dlls to gac again enable the UAC admin mode and restart the server.