PSU Gas Stock to BUY: Shares of Petronet LNG Ltd, a joint venture having 50 per cent shareholding of state-run oil & gas PSUs, gained on Monday as a global brokerage firm upgraded its rating on the oil and gas player. The counter opened in green at Rs 337 on the BSE against the previous close of Rs 333.20. Last seen, it was trading at 336.25 - a gain of around 1 per cent from the previous close.