PO using Gantt

This falls squarely into "helping establish empirical product planning for a complex environment", which is one of the ways that the Scrum Master serves the Product Owner. Not only would I be concerned about a Gantt chart that is attempting to schedule work out 8 Sprints, but also that no Developers have had the opportunity to refine the work and start to estimate or size (if your organization estimates). Such a long-term plan also tends to be poor at showing risks, especially related to things that can't be planned, so there's an inherent risk in showing this type of schedule or forecast to people, especially stakeholders more far removed from the Scrum Team.

Is the Product Owner aware of the missed opportunities here? Multiple inspect-and-adapt opportunities to engage in validated learning are lost -- in a complex and uncertain world -- for the fake certainty promulgated by a Gantt chart.

What should a Scrum Master do? Well, a Scrum Master is good at wondering, and at encouraging people to develop a train of thought. So, is this predictive approach the best way for the value of a complex product to be maximized? I might ask for the Product Owner's help in understanding that.

OK, this is not going to be a popular answer but in some ways it is practical. 

I would let him have his Gantt chart and show it to everyone that he wants.  Then I would work with the Scrum Team to refine Product Backlog Items, plan Sprints, deliver increments ...  all the things that a good Scrum Team will do.  Help the Scrum Team deliver continuous, incremental value.  Help them focus on crafting and accomplishing good Sprint Goals.  Help the team to involve stakeholders as often as possible so that corrections can be made quickly. 

The Gantt chart will become a burden for the Product Owner to maintain, it will become out of date and people will lose faith in it.  It will help others to see the fallacy of planning projects well into the future. It will also help people to appreciate the ability to adapt based upon newly discovered information. 

Thanks folks, I agree with you all.

Daniel, your suggestion is exactly what I was thinking. I cannot "tell" the PO not to us Gantt, if he wants to maintain it and share it with management there is not much I can do except explain the problems with it.

I find it difficult to pull POs who were once managers away from project management thinking, it's not their job, I believe that they would serve the product better by focusing on what is the right thing to do next instead of trying to plan delivery for the developers.

I believe that delivery is not the responsibility of the PO, that's for the developers, if anyone was going to draw up a delivery plan it should be the developers and scrummaster.