I've played for six years, and GNB is my first tank role. I am rusty. I am nervous. But I am very upfront with people and let them know. We all love to learn new roles. I find if you are OPEN with people, they are more than willing and understanding.
As a new tank, I've been pretty much leveling on HoH and doing MSQ Roulette, since there is a second tank, and I can get a feel for things.
I did two dungeons for the first time tanking last night, and for the most part people were really chill.
I totally understand not wanting to spoil dungeons or story before doing them. I just wouldn't take a new job into them. My main is MCH and now DNC as an alt main, so I tend to do new content with the roles I am comfortable with and have down. Once I've got GNB to a skill set I am confident in I will branch out.
Bottom line, be open with your communication. You will always get people who are just looking to rush through and have an experienced tank, but those aren't the norm.
