$$G_{\alpha S}$$ and $$G_{\alpha I}$$ are two forms of the $$G_\alpha$$ protein subunit of the G protein.
$$G_{\alpha S}$$ has stimulatory activity, activating adenylyl cyclase and subsequently inducing the synthesis of cAMP.
$$G_{\alpha I}$$ has inhibitory activity, preventing adenylyl cyclase and thus preventing cAMP from activating any further signalling.
This allows signalling pathways to be controlled in both directions, allowing the activation and deactivation of the pathways.