What are your DIY skills?
GB 42 OEM Swimgrids are way too narrow and weak. I have seen several that collapsed when gently crushed between boat and dock and none that were wide enough to be properly useful.
Yes, I have a bias, having taken the original 21" deep Swimgrid off of my own boat and added 13" to its depth. Mine was better designed than those on the GB to begin with, so all I needed to do was add several new teak strips between the old SG and the transom, and replace the supports. Simple, though I did need to buy several new clamps.
If I was to tackle a GB SG, I would look seriously at building one out of FG. I would go for 30" depth, unless there are davits attached that stick out over the SG, where I would go out another few inches, with a ladder that would be attached to its underside, with several grab handles along the trailing edge. The supports I have are 1/4" flatbar supported by 2" pipe.