Hello from Colorado! New GL500 owner here

Tail lights should be big. They are all that tells an inattentive idiot in a car that you are there at night before he feels something crunching under his wheels.

The tail light on the GL500 & GL650 is the same as on the GL1100, GL1200 and a few other models. They made them big for a few reasons, among them room for 2 bulbs without overheating as well as conspicuity and (most likely) to meet a requirement for a minimum area of reflective material in one of their larger markets without having to have a special light for that market only.

Anyway, welcome to the forum. Please add your bike's model and model year to your profile so that you don't have to remember to tell us every time and we don't have to keep asking when you forget (see Forum Settings link in my signature).

And welcome to the world of antique vehicle ownership (they own us, not the other way around). Your bike is about 4 decades old and (even before being allowed to deteriorate in the sun for years) may or may not have had all of the maintenance necessary to keep it safe & reliable so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage.
I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. You don't need to check the date codes on your tires to know they are old & hard and need to be replaced with something capable of flowing aroiund the irregularities in the asphalt enough to grip. It looks like your bike still has the original rubber brake lines, which should have been replaced every 2 or 3 fluid changes (= 5 or 6 years) so I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid). And don't forget things like the rad hoses and the boot between the engine and swingarm (they can crack on the bottom where you don't see it).

The best advice anyone can give you about customizing any vehicle is to get it safe & reliable in more or less original condition and use it for a while before you start making any changes so it can tell you what changes it needs to make it do what you want/need better. That approach almost always results in something you actually want to keep and use but making changes based on style or on what someone else (who may or may not really understand how the changes affect the way it works) has done often results in a piece of expensive yard art that you can't stand sitting on for more than a few minutes and might even be dangerous.