The gravity of missing Mass on Sundays only applies to Catholics since only Catholics are bound to the Church’s disciplinary precepts, which includes the specific days of the week we worship on.
From this it follows that a Protestant would not be guilty of mortal sin for not attending a church service on Sunday.
With that said, I do think all Christians do have a moral responsibility to worship God and if one were to neglect that duty and not go to a service (no matter what day) due to laziness, it could possibly constitute grave matter.
