Is the Postmasters are Gazetted Officers or not ?? give me information about ranks for Gazetted Officers

Yes post master is a Gazetted officer and they can attest documents for government uses and other uses ,..
Gazetted Officer is a higher level ranked public servant. Authority for a gazetted officer to issue an official stamp comes from the President of India

Post masters of local Post Officer can attest the documents.

Ranks
Class I or Group A (Gazetted)
Example - Officers of the Armed Forces ,Central and State Governments Employees with Group A service rules (IAS, IPS, IFS, IRS etc.), Patent Examiner, Scientists (in government funded research organizations), Vice-Chancellor to Assistant Registrars, Principals and Faculty members of Central and State Universities, Doctors , Engineers and Drug Controller(central and state services), Magistrate and above in judicial services.

Class II or Group B (Gazetted)
Example - Drugs Inspector (state govt service), SDO, BDO, Secretary grama panchayat in kerala, Circle inspector, Tahsildars etc. in State Administration, Principal of Government Schools, junior college teachers, Income Tax and Revenue Officers, Senior and (AAO) Assistant Audit/Accounts Officers of (IA&AD) (CAG) Indian P&T Accounts and Finance Services and Section Officers etc.

Class II or Group B (Non-Gazetted)
Example - Office Assistants, Senior Stenographer, Government School Teachers, Police Inspectors and Foreman,Central Excise and Custom Inspector, Junior Pharmacist and Junior Engineers etc.

Class III or Group C - Public servants in non-supervisory/non-administrative roles, Police Sub-Inspectors.
Example - Head clerks, Clerks, Assistants, Typist, Stenographer, Tax Assistants, Telephone operator etc.

Class IV or Group D- Manual workers (skilled or semi-skilled)
Example - Peon, attender, gardener, driver.