how can accsess gnd in full wave rectifier?
Both DC output terminals of a bridge rectifier will be "hot" relative to its AC input terminals.  If the input is the AC Line, without a transformer, neither side of the output can be connected to AC Neutral or Earth.

You will probably use the negative side of the DC output as your "Circuit Ground", but the whole circuit must be insulated from outside contact as the whole thing will be "Hot".