This is a phase 3, multicenter (when more than one hospital or medical school team work on a medical research study), open-label study evaluating the efficacy and safety of CNTO1959 (Guselkumab) in the treatment of participants with GPP or EP. Participants will receive 50 milligram (mg) guselkumab subcutaneously at Weeks 0, 4 and 12. At Week 16 up to Week 52 participants who will be defined "Very much improved" or "Much improved" in Clinical Global Impression (CGI) will continue to receive guselkumab 50 mg every 8 weeks from Week 20 to the study end (Week 52). At each visit timing from Week 20, participants who will be defined "No change" or "Worsened" will receive guselkumab 100 mg and continue 100 mg every 8 weeks dosing until Week 52. Participants who are "Minimally improved" will also receive guselkumab 100 mg only if investigator considers it necessary. Participants will primarily be assessed for the treatment success. Participants' safety will be monitored throughout the study.