Is braces full-mouthed necessary to correct gapped tooth?!

Yes, and No. It depends on where the gap is. If it's in the front, then you could either leave it be, since some people think slightly gapped teeth are cute, or you can get full braces, which is most likely what's going to happen, or you could just get individual brackets put on, such as 4-5 or so at a time.

Typically they have to shift all your teeth to close that gap. The reason why it is there is because your teeth are too spread out, even if they are perfectly aligned and there are no gaps elsewhere. If your younger, and you don't have your wisdom teeth, then you need something to press that gap shut. All the teeth are shifted.

Of course talking to an orthodontist will give you the right answer... but that's what I've seen, heard, and experienced...