The side material is about .100 thick or 12 gauge. Suppose I could caliper it!Nice trailer. Did you have a hard time finding it, or are they getting back in stock?
About your D rings; my little dump trailer is made from similar material and came with them mounted on 4 corners. You might want to reinforce where they are going with angle; otherwise when you try to cinch something down it will just suck the sides of the trailer in as you are going down the road, causing your straps to loosen. I found that out the hard way one night with a full load of cedar logs.