There are four GAP (Generic Access Profile) roles defined for devices operating over an LE physical transport - Broadcaster, Observer, Peripheral, and Central.
Mynewt OS allows a device to be set up with multiple roles simultaneously. A device can be configured to act concurrently as a Central with one peer and as a Peripheral with another. There is no limit to the roles a device can take on concurrently, provided there is ample memory, CPU cycles, and power. A device playing the Central role also usually takes on the Observer role. A device that is a Peripheral usually plays the Broadcaster role. So technically, a Mynewt device can play all the four roles if a use case so requires.
All mandatory features required in the host and controller parts in order to support all the four LE GAP roles (Broadcaster, Observer, Peripheral or Central) have been implemented.