Hi! First of all, the topic category is set to “Processing” but since it is for the Java based environment, I suggest you to change it to “p5.js” (plus “homework” tag if it is for an assignment). Also the topic title is quite vague (surprisingly, a lot of people come here to ask for help on their code 
) so I recommend you to add a bit more context - like “Need help with p5GUI in instance mode”, for example, which helps other people identify the topic easily.
I took a look at your code and first I found that index.html had some issues like missing tags. For using p5gui in instance mode, the example page shows you how to do it:
To put these 2 things together I have this, which successfully loads the gui. I hope you can take it to the next step from here!
https://editor.p5js.org/micuat/sketches/z4-W18Ksr