GL500/650 Silverwing and 750F also used the same upper fairing as the GL1100 between 81 and 83. As others have noted, the gauge cluster was a Hondaline accessory for this fairing.
I installed this same gauge cluster on my GL500 many years ago. You are missing some parts, one of which will make it virtually impossible to install. When I went about installing, I did a lot of research to determine what I had and how to go about installing it. A Honda dealer (now closed) in the Chicago suburbs had online parts fiche information for parts lookup. I found two that were very helpful and have attached them to this reply.
The key part you are missing is a metal bracket that goes behind the cluster and supports/reinforces the mount to the fairing (#21 in the wire harness fiche). I would find it hard to believe that two screws at the bottom of the plastic cluster housing would secure the cluster to the fairing for any length of time.
Hard to believe, the factory fairing wiring harness had a facility for this accessory (and possibly for other purposes). However, you are missing the electrical wiring harness for this accessory, which has female bullet connectors to accept the wiring from the gauges and plugs into spade connectors just where the wiring harness enters the fairing (labeled SUB-WIRE HARNESS G in both fiche).
Clarion Type II radios had a similar panel, which went in the same area on the fairing as this accessory. However, all the gauges in the Clarion panel were digital, not analog. A full Clarion radio panel would consist of three sections: left – switchable clock or radio frequency display, middle – separate volt and ambient temperature, right – CB radio channel.
I was disappointed when riding at night; the backlighting was very weak for the volt and clock and almost non-existent for the temp and altimeter. Never did find brighter lights with a long enough reach. Before you go about acquiring the bracket, buying/fashioning a wiring harness and drilling holes in your fairing, hook the gauges up to a 12V supply and check the lighting in a dark space after letting your eyes to get acclimated. Unless you are interested in the uniqueness characteristic that will set your bike apart from others.
Hope this helps,
Greg