Unfortunately this doesn't help anyone because it's not as if by deleting gI from your memory you can more easily learn something else, but perhaps it's helpful for a future guide writer or perhaps someone knows why gI is useful.
Edit: The section "The Insane Expression Register" also doesn't make much sense to me. Why would I do this 'system("ls")' thing when there is '!ls'? Using vimscript instead of bash probably has advantages in, y'know, vim, but most people know their shell and probably very few people know vimscript (certainly not me) so I'm not quite clear on the advantages here.