I have some MP3/MP4 files on my ClipV2 which are tagged with track gain but not album gain since they are, well, single files. (Some best-of compilations can be all over the place in terms of levels, too, so album gain makes little sense for those.) If I then set up Replaygain for "album gain" or "track gain when shuffling", no RG will be applied at all, or to be precise: a 0 dB gain. That's annoying if it's one of those nasty tracks calling for -10 dB.
Now I could be wrong, but I do think that I tested RG functionality at some point, and that there was a fallback to track gain if no album gain was available. There definitely isn't with a current build (currently r29659). Or maybe I got that mixed up with Foobar2k, which does implement it?

Would be nice to have then.
Can anyone shed some light on this? Wading through SVN, I
did find /apps/replaygain.c, but was unable to determine where RG information is actually being used / applied.
Interestingly enough, the "Prevent Clipping" function works in any case, but then it may be based on track peak in the first place.