Depends on used version and wanted funciton within the Siebel - in case the digital (non voice) is needed, you have to install the GPlus adapter and Gplus Gateway at least.
As I wrote, at least you need to install the G+ adapter. Recommended is the UCS gateway. All you can find within the documentation for G+for Siebel CRM integration.
Of course you still need the Genesys environment as the background. For SiebelCRM you just need the mentioned components or just follow the documentation where is all described in great details (why asking for so general question on this forum?)