I must confess that I've always been intrigued as to what those old 3-D comics were like, but since I've never really seen an old GA 3-D comic, I really can't judge as to how effective they were at creating the illusion of depth. I have seen a few modern 3-D comics and they are indeed rather impressive... so much so that they motivated me to carry out a few experiments of my own on some of my fan art.
There really wasn't much to find on the internet at the time regarding the actual process that went into making a 3-D comic... at least as it was probably done in the early days of the medium... so my earliest attempts were based on observation (reverse engineering the modern comics I had on hand). But these were mostly unsatisfactory to me, (as I now know) probably due mostly to the complexity of color. Ultimately though, I did find a somewhat obscure book on the subject of stereoscopic drawing... which is the actual terminology for the subject/process.
My experience as someone who has actually tried this is that even with modern computers and drawing programs with "layers" it can be a hassle... with none of these, nearly impossible. So no wonder the early GA attempts fizzled. And I'm just talking about working with pen and ink line art here (art without color).
I suspect that much like with the use of "Poser" and comic book art in general today, most modern 3-D comics are probably done with specific computer software that renders the work child's play.