As for digits of significance, neither "8.5" or "8.50" are representing the number of significant digits. Replay Gain is stored with several more digits than displayed. However, it's hardly worth showing "8.4997632" just because that's what we have stored.
I was only refuting your contention the trailing '0' is superfluous. I do understand the meaning of "significant digits" and was not trying to suggest you should be displaying all of them. When reporting a number, one decides
how many significant digits to include. I don't believe it's improper to refer to those as a "number of significant digits."
What you've gone on to describe is a limitation in your display system. I don't know why an integer wouldn't be displayed as an integer and a decimal as a decimal. I'm just pointing out that if you display a decimal with two
digits of significance (hopefully rounded correctly), trailing zeroes should not be dropped. And in a column, the numbers should be right-aligned.