What would happen if trump were to issue an executive order banning the us supreme courts ruling on gay marriage?

Nothing would happen. The court wouldn't follow the order. The ruling would stay the same.

The SCOTUS doesn't work for trump.

The country would laugh at the idiot for believing he had such authority to do so.

That is not within a president's power.

You don't know how this stuff works, do you?

Nothing. Absolutely nothing.

That is something I don't believe an Executive Order can do in the first place.

it would be invalidated as only the supreme court can change a previous supreme courts decision

so dont waste your breath 

The US Supreme court would strike his decision down.