In the Giza Plains during the rainy season there are rotten trees spread around the nearby vicinity that you have to cut down. They float down river to where the driftwood is to help form a bridge across. You'll know you've got all the trees chopped down when you hear a little chime sound. Then you can cross over to where the Gil Snapper is. Just to let you know however, it needs to be pouring rain or the mark won't appear. Just keep zoning in and out of that area until it does and the Seeq will come up to you and tell you when it appears.

And yes the Antlion Hunt is assigned to you by Montblanc. Just go to the Hunter's Guild and talk to him and you'll see the hunt appear on the selection board.