Gearing , how does it work?

[Rufalus]

or the lvl 50 dungeons and raids are now obsolute and usless to get in as the gear will be replaced by quest gear in havensward?

That's right, the lv50 gear from a stats/gearing perspective is now obsolete and will be replaced by gear in Heavensward. You just take what you are given from the main scenario but don't dwell on lv50 content too long outside of that -- continue to the extended story in Heavensward where you will continue to be given higher gear from main scenario quests and side quests.