ADDITIONAL SYSTEM INFORMATION :
Gentoo

openjdk 13.0.1 2019-10-15
OpenJDK Runtime Environment (build 13.0.1+9)
OpenJDK 64-Bit Server VM (build 13.0.1+9, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
Even the HelloFX.java refuses to launch without X. On a wayland only platform, java is unable to even find an appropriate display:

GDK_BACKEND=wayland java -Djdk.gtk.verbose=true -Djdk.gtk.version=3 --module-path /home/amitprakash/swing/javafx-sdk-13/lib/ --add-modules javafx.controls HelloFX

Exception in thread "main" java.lang.reflect.InvocationTargetException
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.UnsupportedOperationException: Unable to open DISPLAY
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$new$6(GtkApplication.java:173)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:312)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.<init>(GtkApplication.java:171)
at javafx.graphics/com.sun.glass.ui.gtk.GtkPlatformFactory.createApplication(GtkPlatformFactory.java:41)
at javafx.graphics/com.sun.glass.ui.Application.run(Application.java:144)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.startup(QuantumToolkit.java:280)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.startup(PlatformImpl.java:269)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.startup(PlatformImpl.java:158)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.startToolkit(LauncherImpl.java:658)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:409)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
... 5 more