otherwise, y'know, maybe I wouldn't have mentioned srec_cat in my original post, eh?
I genuinely overlooked that, sorry.
denigratory vitriol
That was not the intention.
The long version would have been, perhaps:
Don't waste your time.
Binutils are notoriously overcomplicated, with some of the features gotten there either very soon and then simply never been properly maintained for lack of mainstream need, until gotten unusable for some other related and more important development; or as some vanity/marginal feature forming part of some particular effort for some very particular target/toolchain. Those features then function weird, depend on circumstances (e.g. target, host, (non)existence and particular naming of some particular section, etc.) and are incompletely/insufficiently/unusably documented.
Attempting to use those features then leads to frustration, as they often "almost work".
Been there, although not with --gap-fill. For example, I've tried the linker-script version but as others mentioned, it's not it either, so I quickly gave up. Wasted substantial time on trying to comprehend orphan sections and actually trying to utilize them for "no need to update linker script" approach, just to having that failed miserably and having to rework that build setup after linker/toolchain update (forced by client requirement). Sucessfully using injection of binary data tables through converting bin to elf to be linked together with "normal" objects in one project, but that too was a massive waste of time to get it working with walking down quite a couple of dead ends (with the repeated lure of "oh this time it almost perfectly worked"), and I won't update the linker for that project, ever. Etc.
To find out the idiosyncracies and real working of such features, often the "real documentation" (i.e. sources) have to be consulted, which, needless to say, is a painful experience in itself, for the same historico-layering reasons.
srecord is so much, much, much more fit to this particular task, even if its documentation is ehm weird.
JW