The electronic property of GeSbTe compounds is studied within the notion of their topological insulating phase. We show that these compounds, as pseudo-binary alloys of GeTe and Sb2Te3, in their crystalline phase with well-ordered layer stacking have imbedded surface-like states originating from topological insulating Sb2Te3 at interfaces between GeTe and Sb2Te3 layers. We demonstrate that these conducting states can significantly affect the electrical conduction of the compounds and are sensitive to Ge migration. Using first-principles methods, we show that the topological insulating phase of GeSbTe compounds can be manipulated by controlling the composition and/or layer thickness. It is found that the interaction between the surface-like states intervened by insulating GeTe layers creates an inversion gap to turn the compounds into topological insulating phase. We anticipate that the topological insulating property of GeSbTe can provide new insight into understanding the underlying physics in structural transitions of the compounds and a perspective of utilizing phase change materials as spin-control devices as well as non-volatile memory devices.