What food and potions should I be using on GNB?
Highest level strength potions available. Always use HQ. Food will vary from tier to tier based on what foods are available, but usually it will be Crit/Det food or Crit/Skill Speed food.
Should I be using X opener if Main Tanking (called Primary Tank in some regions)??
Being Main or Off Tank (Primary or Secondary Tank) does not affect what opener you use. You will almost always be using the standard raid buff opener (see #gnb_resources) unless there is a fight specific opener for a fight.
Gnashing Fang is ready earlier and earlier before No Mercy, why is this happening?
Do I hold Bloodfest for No Mercy?
See the Bloodfest part of the main GNB guide.