The description of a gesture requires temporal analysis of values generated by input sensors and does not fit well the observer pattern traditionally used by frameworks to handle user input. The current solution is to embed particular gesture-based interactions, such as pinch-to-zoom, into frameworks by notifying when a whole gesture is detected. This approach suffers from a lack of flexibility unless the programmer performs explicit temporal analysis of raw sensors data. This paper proposes a compositional, declarative meta-model for gestures definition based on Petri Nets. Basic traits are used as building blocks for defining gestures; each one notifies the change of a feature value. A complex gesture is defined by the composition of other sub-gestures using a set of operators. The user interface behaviour can be associated to the recognition of the whole gesture or to any other sub-component, addressing the problem of granularity for the notification events. The meta-model can be instantiated for different gesture recognition supports and its definition has been validated through a proof of concept library. Sample applications have been developed for supporting multitouch gestures on iOS and full body gestures with Microsoft Kinect.