Right now the Gantt charts are basic to the point of being (possibly) less than a MVP tool. For example, there appears to be no way to create dependancies to a milestone, such as a ship date. Why not? This is somewhat related to https://big.ideas.aha.io/ideas/APP-I-2052
We have made ample upgrades to the Releases Roadmap page, and plan to continue making upgrades over time, as we generate feedback from the community.
In regards to your specific request to create dependencies to milestones, it is possible to do this by hovering on the gray dot of the phase of interest and drag it to the associated milestone.