Can I operate with non-integral (fractional) values? For example, I'd like to output the difference between album and track replaygain, but I cannot substract one from another because $sub operates with integer values only (while RG is fractional most of the time).

If TAGZ doesn't do floating point, you can fake it.

Pull the number apart at the . and do the math for each side separately, then display the two results with a . in the middle. It'll take a little figuring to come up with the right algorithm to account for carrying-the-1-type issues.

$replace(%__replaygain_track_gain%,.,, dB,)