Can someone tell me how to orientate the gatehouses? I have tried everything I could think about doing.

I just started playing Stronghold HD -- so I'm not sure if it applies to the other games of the series -- but in the build menu when you click gatehouse it opens a new submenu with four gatehouse options (which are the gatehouse in different directions).

You can't do it manually, the only way it's like SheBear says. When you choose the gatehouse a sub menu pop ups and you can choose the direction according to your needs or the direction of the walls.

I don't think you should try to change the gatehouse's orientation. It was born that way and should have the same rights as any gatehouse.