You will find your first Giganto at the Weasand of Cados. In these winding passages you will eventually get to a section where the upper path will let you carry on with the story and the lower path will lead you to a Giganto. It is pretty obvious from the size that it is not a regular monster and you won't run into it by accident. If you somehow do manage this, I would suggest running away. Unless you've been grinding chances are you won't be levelled up enough to defeat it the first time you are in this area. However, you will return through the story a couple more times and it is usually accessible except for a short while. Make sure you've got plenty of items and are nicely levelled up and you should have no problems. My memory is that it is around level 36.