The weight isnt enough? If no I would suggest fixing some mesh to the underside of the slabs, not the full size just a couple of pieces 300x300 or so along the length. Cut so that there is 50mm or so of wire free of the mesh all around the mesh. Fix it to the slabs with U nails and bend the cut ends down. This will work like velcro and if you can get a piece of box or bar in there you can bend them around the gabion mesh. That way the top is left without fixings and all those glorious splinters can find homes in someone's arse.
Alternatively weld some flat with pins pointing up. Fix these to the gabions and drill partway through the slab to coincide with the pins. Repeat splinter comment here.
Alternatively weld some flat with pins pointing up. Fix these to the gabions and drill partway through the slab to coincide with the pins. Repeat splinter comment here.