My grates are the same size and I used 1x1x0.125 angle and #12 flat expanded metal for the grate. I did not add any center bracing and they are plenty strong enough and do not flex any. The angle iron was positioned so the "flat" is on top so as not to have an edge on the grates and also to allow me to grab the bottom lip if needed to slide the shelf. Only other thing I might mention is that I put the expanded metal on the bottom side of the angle iron "flat" so that all the cut ends/ears from the expanded metal are hidden and do not need to be welded down and ground off. It looks a little cleaner as well.
Mine grates are a little larger on my GF x 5 grates, and I did not add a center brace either and it works fine. I also used 1x1x1/8" angle on mine. but I used 1/4x1 flat bar on the front edge. That way it gave me an extra 1" clearance (3/4" actually)/ plenty stiff especially once the #9 expanded metal was tac welded to the frames. 1/8" x 1 flat bar would have probably been fine. but I had teh thicker stuff laying around, so I used it..