If you are asked to do so, then proceed to the next step. It will automatically go with the gmail account if you have signed in Gmail already or sign in with your existing account. You do not need to create new Gmail account for this. If you still find difficulty, share that problem with a screenshot so that we will able to give you accurate solution.
Partha, you no need to add all the sections in your Adsense account in order to earn Adsense revenue. You need to focus on contents which get traffic. Forum is not a place for earning adsense revenue. You can try out Articles / Ask Experts (already you are doing) / Schools / Colleges / Question Papers / Jobs. I say it from my experience (I have earned $7000/- so far from ISC alone since last 3 years, mostly from Jobs and Articles Section, now job section is almost dead for Adsense revenue)
Ravi, I have visited your profile. You have mostly contributed to Forum section during November 2016. It is very difficult to earn Adsense revenue from Forum section. Even if you contribute big in Forum, you won't be able to earn much.