When you run ./gradlew build, it builds EVERYTHING. This means debug and release builds for desktop and all installed cross compilers. For many developers, this is way too much, and causes much developer pain.
To help with some of these things, common tasks have shortcuts to only build necessary things for common development and testing tasks.
For projects wpiutil, ntcore, cscore, hal wpilibOldCommands, wpilibNewCommands and cameraserver, a testDesktopJava and a testDesktopCpp task exists. These can be ran with ./gradlew :projectName:task, and will only build the minimum things required to run those tests.
For wpilibc, a testDesktopCpp task exists. For wpilibj, a testDesktopJava task exists.
For wpilibcExamples, a buildDesktopCpp task exists (These can't be ran, but they can compile).
For wpilibjExamples, a buildDesktopJava task exists.