Sorry to hear of your troubles.
Used to be that all the positive cables were always red. This seems to have gone away for our older cars, likely to cut costs for the suppliers. Sucks. I put a piece of red heat shrink tubing around the positive cable just for my own peace of mind, but that's not something that easily available to everyone.
I would suspect that you have killed the gauges. The fuses won't care about the polarity, but the gauges might. Use your multimeter to test the fuses one at a time. A fuse does not care about the direction of current, only the amount of current. Anything electronic-ish will be vulnerable to reversed polarity.
I don't know what the cutoff is, but some time in the '80s Jeep changed the dash design for these cars. Before the change will have an ammeter, after the change will have a voltmeter. There are wiring diagrams specific to the '87 here -
https://oljeep.com/gw/elec/GW_wiring.html
The first section shows a voltmeter, which indicates the later design. There is a complete '89 technical service manual (TSM) on that site, free to read and download, that you can use to evaluate the gauges. The '89 also has this later style dash, and the procedures and specs in the text will be the same.
You test the gauges in two parts. Disconnect the sender and measure the resistance of the sender to ground. Use a fixed resistor between the sender lead and ground to test the performance of the gauge.
Your catastrophe at the time was probably the alternator acting like a direct short when faced with reversed polarity. I have never seen someone reverse the cables like this, but this outcome seems likely. This probably melted one or more fusible links in the wiring harness. Looking at the scrap above of the wiring diagram, I would expect the clock to be dead (VFD type), the low fuel module, the gauges. Anything called "module" likely contains integrated circuits, which will go poof in response to reversed polarity.
I wrote this article about gauge testing on a CJ Jeep:
http://www.earlycj5.com/xf_cj5/index.ph ... er.143458/ This will show you the
concept in more detail, but the specifics will be very different for your GW.
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