Dear, that gas is SO2( sulphur dioxide gas), This gas has a great reducing property that it can reduces the coloured solution in to halides. And that become colourless. In bromine solution it Br2 become into HBr after passing SO2 in it ..just for example if u take Br2 with water and passes the SO2 gas in it it will turns into HBr.
Hope it helps.. thank u dear...