Ok... I have researched a bit more... Since Minecraft Forge 1.13.2-25.0.28 (released 5 hours ago), after running gradlew eclipse, we shall run gradlew genEclipseRuns
This new gradlew task will create "runClient.launch" and "runServer.launch" configurations. But it is not necessary to import them. The task already does this automatically, including them into the Eclipse project.
I've just tried myself, and it works as it should: the mod is compiled/built, Minecraft runs, and the mod is loaded.
I can't guarantee that it will be the same tomorrow (we had 8 Forge releases this Monday!)