Gradle is a java build system. Many java projects contain a ./gradlew script you can run without needing to actually install gradle manually. This is also useful for getting the lastest version installed.

If you're using a non-default java install for the build you need to set the JAVA_HOME environment variable. It should be a path to the java install location (not the bin folder itself).

gradle build:     builds the project
gradle tasks:     list all the gradle subcommands for this project
gradle runClient: launch minecraft client