How can gravity prime a pump.....

...with the inlet facing downward?
A friend of mine has the inlet facing the ceiling which made sense to me since liquid would flow directly into the pump.

March says to orient the pump with the inlet facing downwards, meaning liquid has to flow down from the Kettle/Tun/HLT then turn 180 degrees to get into the inlet to prime and them be pumped out.

This is right?

When priming , do y'all leave the outlet ball valve open or closed?

Inlet should face down- as you "flood" the pump with fluid, the air bubble will rise up and out of the pump. This will prevent cavitation.

But I've also successfully placed my pumps horizontally and that worked well, and that is the way I have them currently. They are head down, mounted horizontally. I have few problems with priming.

I leave the outlet ball valve open, so that the air can come out.

I didn't believe it until I tried it.

doesn't matter how many ups and downs you have as long as they are below the top outlet and the air can get away, liquid will seek the lowest level eventually..

Liquid is like a snake... it seeks the path of least resistance, the weight of the liquid and gravity pushes the wort to the pump. As long as your pump and all lines are below the vessel you will have no problems getting flow.

Yes leave the valve open but also make sure ALL valves in the line are open to allow any air to escape. Definitely make sure you don't have any relative high spots in the line because you can form an air lock. You shouldn't have any problems if you do both of those things.