A Gantt chart displaying multiple jobs (and associated phases) can be created from the main job list. Tick the box next to each job you wish to include on the chart and change the view type from List to Gantt:
You can then interact with the data as normal within the Gantt UI - drag and drop, edit dates & durations, add phases / stages (depending on system setup).
You can also display a selection of phases from multiple jobs in a Gantt - useful for seeing all the phases of a certain type in one place. Use the same Gantt button as above on the phase list:
Either select from the list displayed, or use a view / filter to display the phases you are interested in and select all then click to show as a Gantt:
Note that this view is for display only, you cannot move or adjust a phase in isolation from the other phases on the job.