You can also show relationships between multiple projects by linking tasks across projects. To set such dependency,

Go to your project on which you need to create a dependency.
Select the Gantt
Select the task for which you need to add a predecessor and edit it (press e on your keyboard).
Navigate to the Predecessors tab.
Click on Add. Under the Project drop-down, select the project and then Task who you wish to set as predecessor for this task.
Set the Type of dependency and/or the Lead/Lag time.
Click on Save.

NOTE: The dependency will NOT be shown under the 'Predecessors' column.

To view dependencies between projects:

Create a report of type 'Task' with output type as 'Gantt'. For steps, refer here.
The Gantt part will show you the dependencies as seen in the below screenshot.

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