While filing form for gst application, should i fill 'taxpayer' or 'other notified person' in 'i am a' section?

You have mentioned that you are filing the ITR regularly, if so you certainly would be filing the returns through an auditor, if so, then what prevents you from asking this petty doubt from your auditor, which is more a practical issue instead of any legal issue. 

He need to have an auditor only if he files return under Profits and Gains from Business or Profession. and  his income is more than Rs.50 lakhs per annum or if he claims more than 50% of the income as expenditure. He has to say in the GST form that he is a tax payer. Filing ITR is always directly and not through auditor. The auditor only uploads certain forms before the filing of returns by his client.

If you want to trade (online) regularly pay GST and endorse it accordingly, as taxpayer and for part time you may submit other as notified person