Who is the one...sounds a little stiff. Is there anything wrong with:
Who is that standing by the gate?
That's a good translation, though I prefer using 'at' here. The extra clause is needed in Hungarian when you make expressions like that, but it sounds odd in English. Literally it would be "Who is that who is standing at the gate?"
Sure, it's a translation. But it can work just as shortly in Hungarian: "Ki áll a kapunál?"
I'd prefer the translation that Daniel suggested above, to at least hint at the use of the extra clause.
It sounds a bit weird in English, but it seems acceptable. I would recommend "Who is it who stands at the gate?" or "Who is that standing at the gate?" instead if you want to preserve that structure.
Can someone please explain the difference between "mellett" and the suffix "-nal"?
Mellett means "next to" or "beside". It is direction-specific, referring to an object that's left or right of something else. On the side.
The suffix -nál/-nél means "at" or "by". It's not direction-specific but only refers to the immediate vicinity of an object.
If you're talking about an object that doesn't have specific sides, like a tree, then -nál and mellett have the same meaning. But if something has defined sides, like a car, then "az autó mellett" would mean that you're somewhere by the doors, while "az autónál" just states that you're somewhere close to the car.