Welcome to the site.
1 Depending on cost and how it works
2 Laminate would not go directly on dirt or gravel.
3 You often get plywood that is warped, even with frames as in the site you posted, you could have corners the are high or low creating tripping hazards.
I would do what we call a floating floor, Have the floor leveled with 1/2" minus crushed stone and compacted, make sure it is above the elevation of the ground outside the barn.
Cover the gravel with 6 mil poly and then lay down 2x3s @ 16 inch on center with as long of 2x3s I could get or 2x4s as they will more saleable. At the joints just run the next one past by a foot
Then over that run 5/8 tunge and grove plywood, you can get a better grade that is sanded and paintable and put it down with screws with a flush setting on the screw driver, offset the joints on the plywood. No glue!