SMP is a mobile platform, GW is not. See GW as a modern SOA / WS that exposes your BAPIs as oData. SMP is the platform for you mobile apps. If no platform is needed, SMP is also not needed. FOr instance, there are some mobile apps from SAP that use GW but not SMP (SolMan apps).
Most of the SMP apps that use oData can be run without SMP, it's just that the mobile platform from SAP is SMP. For instance, the oData mobile apps in SUP 2.x do not really use SUP, they talk "directly" with GW.