The best lead I have so far is that this is some kind of unexpected parsing; see https://stackoverflow.com/questions/38895509/jenkins-pipeline-no-such-dsl-method. However, none of the specific answers on that page seem to apply.

I assume the fact that runDir is a GString is not relevant, since the Groovy docs say GStrings can always be passed to methods expecting Strings.

It turns out it really was the (lack of) GString to String conversion causing joinPath to not match as a valid function. Perhaps something breaks for varargs...

Kian-Tat Lim, how should we handle the final review, given that I've merged a bunch of changes already (and should probably test the "final" version as well)?

Thanks for your help. Unfortunately, it appears that there's another problem: the gen3_to_job.py call doesn't produce any output, so nothing gets uploaded. I'll open a new issue (DM-29066).