Okay, so I downloaded a new version of Forge. I did cd <forge directory>, and then ./gradlew setupDecompWorkspace. After it was done building, I looked in build, and saw there was a natives directory (yay!). After launching NetBeans, opening the Forge grade project (using the Gradle plugin) and copying my packages over to the new project, I configured the command args for "Run", setting them to "runClient." I clicked "Run," and it launched!
But it was too good to be true. I closed it, did "Build and Clean," then "Run." To my surprise, I got the familiar error about libraries. I checked back in the build directory, and (gasp), the natives folder had vanished! So question: Is Build and Clean the culprit?