Thanks everyone for your replies. After a lot of research, I found the most economical solution. I used QuikSteel Plastic Tank sealant. I cleaned the surface around the gauge really well using vinegar and an old rag. Next, I proceeded to spread the sealant around the entire perimeter of the gauge, leaving the gauge itself open and still able to be read. I have two of these 1980 Suzuki FA50's and neither are dripping a single drop at this point after both of them having the same leak issue. Really happy with the QuikSteel and for $10, it was an inexpensive fix.
Not surprised that the 1980 was the only model to have the gauge on the gas tank. My guess is that they had a lot of issues with this and decided to remove it. As for now, we're all good. If anything changes, I'll update.