GTK_IM_MODULE=xim

[gusl]

For future reference, the above is how I got Ubuntu to obey my Compose file, which customizes the mapping between keyboard input and characters (in particular, for making accents, cedilla, etc), since I've been spoiled by too many years of the "US International" keyboard on Windows (a similarly-named keyboard exists on Linux, but it's best suited for Eastern European languages, e.g. '+c produces ć).

Next time I solve a long-standing problem, I will make better records.

It's a really obscure setting. I owe this knowledge to a random volunteer on FreeNode.