does anyone know the best technical resource on the net available on the subject of why gaps have been so difficult to get rid of?

No, but we can explain here. This is also paraphrased in the empeg FAQ as well as in the Readme file for my Gapkiller program:

1) MP3 files were never meant to be played back-to-back seamlessly. When they designed the MP3 specification, they simply didn't think of this.

2) Each frame of an MP3 is a fixed timespan. You can't have "half of a frame". So, no matter what, all MP3 files always have their last frame padded with a certain amount of silence.

3) MP3 encoders have some sort of a strange thing where the very first frame almost always has some silent padding at the beginning. Note that this is unrelated to #2, above. I don't understand the technical details of this one. Maybe it's got something to do with the encoder needing to "spool up" and fill the bit-buffer or something. I don't know exactly. I actually e-mailed Fraunhofer about this one and they basically said, "yeah, it does that. It's normal. There's nothing we can do about it."

4) MP3 frames are interdependent. They use a "bit reservoir" which allows a later frame to use some of the data from a prior frame. So frames within a file can play seamlessly against one another, but frames from two separately-encoded files cannot.

5) Playback programs almost always insert additional silence of their own. They do not properly spool the data so that the two files can play without a hiccup between them. The 2.0b7 car player software is very good about this and does not hiccup between two files. However, the MP3s themselves still have the encoded silence (above), so there is often still a hiccup.

Have you read the FAQ entry and/or tried my Gapkiller program? I can get Dark Side of the Moon playing perfectly on my car player now, except for one particular song transition where there is a very slight hiccup.