It is important to understand that the Sametime Gateway does not have a deployment plan like the other Sametime components. This means you do not begin the install from the SSC. You must install it from its own installation program. Once the installation is complete, you can register the Sametime Gateway with the SSC and administer the Sametime Gateway from there or install it with its own Deployment Manager console.
For further information on how to connect the Sametime Gateway to the SSC, refer to the IBM Sametime 8.5.2 Installation, Migration, and Configuration Guide at: http://public.dhe.ibm.com/software/dw/lotus/sametime/st852/st_852_pdfone.pdf.