Gradle is a build system. Build systems are software tools designed to automate the process of program compilation. Build systems come in various forms, and are used for a variety of software build tasks. While their primary goal is to efficiently create executables.
Another related term is Build automation which is the process of automating the creation of a software build and the associated processes including: compiling computer source code into binary code, packaging binary code, and running automated tests.
Few similar build system for other languages are (see complete list here):