the nice thing about the GMC dash is that the gauge holes will likely accept an aftermarket panel gauge or a pod gauge that is removed from the pod/pot that it comes with. I would actually say, by looking, that some of your gauges have been replaced already that way. since the temp and amp gauges have what looks like the stewart warner emblem on them. the tach is also something new to the old dash but I can't make out the emblem, is it putco? for the speedo you can likely also find a panel gauge to fit and get the correct speedo adapter from a local or online speedo repair shop. you may be able to find an electronic gauge to work directly with the new speedo. I would say, personally, to save your money and don't bother having the old speedo fixed because it would mean a speedo repair cost plus an adapter to make the cable driven speedo work with the electronic output from the trans. maybe look at a gps speedo because that way it doesn't matter if you change tire size etc. not sure they work well in nasty weather if it can't get a clear view of a satellite. maybe someone could chime in who has had the experience with those.
lots of guys cut their own dash panels for the GMC dash because they are simply flat sheet. you could even use flat plastic sheet because it is easily worked with hand tools likely found in the home shop. plastic is pretty cheap if you don't have a piece of something laying around. even a piece of wood paneling would suffice for a pattern and to see if you like the look of a new gauge set layout. if that becomes your plan be sure to have your steering wheel of choice in place so you can lay out the gauges in a way that makes them easy to see past the steering wheel. there is also some cool vinyl that could be applied over your "new" dash to make it look like you want without painting etc. I have seen one truck that used the dash cluster from the donor vehicle in his GMC and that enabled him to use the original donor vehicle sending units etc. lots of guys have installed complete dash panels from cars etc, but that is a lot of work plus the front glass needs to come out for access to the spot welds OR the new dash would need to be welded to a stub of the old dash up close to the front glass. never good with the glass in anyway.
https://www.speedhut.com/about-speedometers.html