Processing already has several GUI libraries, the most commonly used (based on number of questions I see in the forum) are controlP5 and G4P, both are long established and well supported libraries. G4P was first released on 22nd March 2009 so will be celebrating its 10th birthday soon, and controlP5 has been around longer still.
Now I must make a confession, I am the creator of G4P so am a little biased but G4P also has a visual design tool called GUI Builder. Both of these can be installed via Processing’s contribution manager also my website has extensive support for both including video guides to using GUI Builder.