Qlik Help says: "Extensions are not supported for apps published to Qlik Cloud Services. Visualizations using extensions cannot be opened in Qlik Cloud Services. The app consumer will receive a message indicating that the visualization was not found."
You asked about using our Gantt chart extension for Qlik Sense in the cloud. As of February 2020, customers using Qlik Sense in the cloud can benefit from 3rd party extensions. So I am glad to tell you AnyGantt Extension is now available in the cloud!
For step-by-step instructions on how to download and install AnyGantt Extension for Qlik Sense in the cloud, watch our quick video tutorial we've just made public: