Gulet ship is by default quite comfortable, large and massive vessel. This also means this shipa are surprisingly stable. But you have to know they are still vessels so there will be some small movements while you are on board.
In case of bad weather captain will always advice the clients to stay in port until the weather gets better and if clients listen to the advice, there is NEVER ANY PROBLEMS.
However it can happen clients have hard times believing to the Captain and they insist to go out of safe and secluded bays and ports to rough seas.
If the captain will see it is not dangerous (only unpleasant) – he will go out. Because sometimes clients just have to see personally what is out there.
So the bottom line is, if you will listen to your captain, you will not experience any problems and you should not get seasick.