Suleman didnt attach to a Gursikh. The guy who initially came to Sant Isher wasnt a gursikh. He also was quite young. He was hanging around the grave and i think he disrespected the grave. Theres alot of small detail.
And ye u are right. Ghost cant just attach to anyone who does naam japna. These things are there but they only affect us if we entertain them. Best thing is ignore them and jap naam. That way Maharaj will protect u.