Is ghusl obligatory for one from whom semen was discharged involuntarily and without desire, due to his suffering from semen retention for more than two and a half months, and the resulting psychological and physical pain?

What occurred from you is masturbation, which is the emission of semen without intercourse. You must seek Allah's forgiveness and repent to Him, and you must take precautions for safety and caution, and distance yourself from causes of suspicion and allurements of temptation.