Depending on various parameters like listing on ISE, GS, revenue, and income, PSUs in India come under 3 categories :
Maharatna
Navaratna
Mini-ratna
You can fill their forms before and after the Gate application as the date of their applications are not fixed.
Some maharatna companies are:-
Bharat Heavy Electricals Limited (BHEL)
Coal India Limited
Gas authority of India limited (GAIL)
Indian Oil Corporation Limited (IOCL)
National Thermal Power Corporation Limited (NTPC Limited)
Oil & Natural Gas Corporation Limited (ONGC)
Steel Authority of India Limited (SAIL)
Bharat Petroleum Corporation Limited
Some navarna companies are:-
Bharat Electronics Limited (BEL)
Container Corporation of India Limited
Engineers India Limited
Hindustan Aeronautics Limited
Hindustan Petroleum Corporation Limited
Mahanagar Telephone Nigam Limited
National Aluminium Company Limited
National Buildings Construction Corporation Limited
NMDC Limited
Neyveli Lignite Corporation Limited
Oil India Limited
Power Finance Corporation Limited
Power Grid Corporation of India Limited
Rashtriya Ispat Nigam Limited
Rural Electrification Corporation Limited
Shipping Corporation of India Limited
Some miniratna companies are:-
Miniratna Category - I CPSEs
Airports Authority of India
Antrix Corporation Limited
Balmer Lawrie & Co. Limited
Bharat Coking Coal Limited
Bharat Dynamics Limited
BEML Limited
Bharat Sanchar Nigam Limited
Bridge & Roof Company (India) Limited
Central Warehousing Corporation
Central Coalfields Limited
Chennai Petroleum Corporation Limited
Cochin Shipyard Limited
Dredging Corporation of India Limited
Kamarajar Port Limited
Garden Reach Shipbuilders & Engineers Limited
Goa Shipyard Limited
Hindustan Copper Limited
HLL Lifecare Limited
Hindustan Newsprint Limited
Hindustan Paper Corporation Limited
Housing & Urban Development Corporation Limited
India Tourism Development Corporation Limited
Indian Rare Earths Limited
Indian Railway Catering & Tourism Corporation Limited
Indian Renewable Energy Development Agency Limited
India Trade Promotion Organisation
IRCON International Limited
KIOCL Limited
Mazagaon Dock Limited
Mahanadi Coalfields Limited
Manganese Ore (India) Limited
Mangalore Refinery & Petrochemical Limited