The BSE benchmark Sensex today gained 124 points in early trade on sustained buying in ONGC and RIL as government has virtually deregulated diesel prices in a key economic reform measure. The 30-share barometer rose 124 points to 20,087 with stocks of oil and gas, auto, PSUs,metal and banking sectors extending support.Similarly, the wide-based National Stock Exchange index Nifty moved up by 37 points to 6,076.