Short-circuit dependency chain if task outputs exist

I think I understand now what you want your build to do:
skip the generate tasks when the output file of the generate task already exists.

I am not yet sure if you need the output of the download task as well, even when you have the output of the generate task. My suspicion is that you don’t, so you may want to skip it as well when the output of the generate task is present.

Gradle has a way to skip tasks (i.e. don’t execute them depending on a condition): Task.onlyIf(). See the user manual for more information.

In your case, you can add the onlyIf condition to the generate and download tasks:

download.onlyIf { !layout.buildDirectory.file(name).exists() }
generate.onlyIf { !layout.buildDirectory.file(name).exists() }

Cheers,
Stefan