Provide envvars to a GradleBuild task

I jumped on the Gradle train back with 1m8. Back then, I already had to call an external Gradle project. I used a JavaExec task for that. Now I want to move to the proper way and use a GradleBuild. My problem is, I can’t seem to find the way to provide additional environment variables like I could with javaexec.
How would I do that for GradleBuild?