Really? That's a bit sad, that there's a whole major version of MC that's just... missed... because Forge was still figuring out its big rewrite.
I'll document the work-arounds I've found here, in case anyone else in the future stumbles across the same issue:
"gradlew build" and "gradlew runClient" work from a fresh cache. But they fail after running "gradlew eclipse". The issue is that either the deobfuscated source jar or recompiled jar (I haven't narrowed it down yet) cause the "Error getting artifact" issue. Deleting those from the cache directory makes "gradlew build" work again, but then you don't have deobfuscated sources to work with, which makes development... more exciting.
Anyone trying to get 1.13.2 working is probably best served by starting from 1.14.2 and then back-porting.