How do I add default JVM arguments with Gradle

From the top of my head I can think of 2 options:

Option1: Do what @Ethan said, it'll likely work:

package placeholder;

//your imports

public class Application{
  static {
      System.getProperties().set("javafx.embed.singleThread", "true");  
  }
  // your code
  public static void main(String... args){
    //your code
  }
}

Option 2: Use the application plugin + default jvm values

build.gradle:

apply plugin: 'application'
//your code
applicationDefaultJvmArgs = ["-Djavafx.embed.singleThread=true"]

Now you can run your code 2 ways:

From gradle

$gradle run

From distribution(script). from the generated script that the application plugin will provide:

$gradle clean build distZip

Then gradle will generate a zip file somewhere under ${your.projectdir}/build. Find the zip then unzip it and under /bin you'll find a ${yourproject}.bat and ${yourproject} executables. One is for Linux/mac/unix (${yourproject}) the other one is for windows (${yourproject.bat})

Option 3 (Android Developer): Use gradle.properties to set jvm argument

# Project-wide Gradle settings.

# IDE (e.g. Android Studio) users:
# Gradle settings configured through the IDE *will override*
# any settings specified in this file.

# For more details on how to configure your build environment visit
# http://www.gradle.org/docs/current/userguide/build_environment.html

# Specifies the JVM arguments used for the daemon process.
# The setting is particularly useful for tweaking memory settings.
# Default value: -Xmx1024m -XX:MaxPermSize=256m
# org.gradle.jvmargs=-Xmx1024m -XX:MaxPermSize=512m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8 

# You can setup or customize it according to your needs and combined with the above default value.
org.gradle.jvmargs=-Djavafx.embed.singleThread=true

For more info on how to use gradle build environment on docs.gradle.org

applicationDefaultJvmArgs is provided by the Application plugin. So if you apply that plugin, the error would probably go away, and you should be able to execute the program by issuing gradle run once you set the mainClassName property to the fully qualified class name, the main method of which you want to invoke.