OK so I solved this by specifying the version to run inside of the Info.plist file.
To access that right click on the OSx JD-GUI icon and go to "Show Package Contents" then navigate into the "Contents" directory and open "Info.plist" inside of a text editor.
You want to look for the line that looks like:
<key>JVMVersion</key> <string>1.7+</string>
I believe the issue I was experiencing was caused by having Java 10 installed so the real issue may be that High Sierra is OK but Java 10 causes problems. To fix that I specified the Java 7 runtime in the Info.plist file by changing the above line to the following:
<key>JVMVersion</key> <string>Your-java-version</string>
This allows me to run the JD-Gui from the osx .app wrapper as I did before my update.
Command for checking java version:
java -version
