How R and G can be zero simultaneously in lossless transmission lines?
R is the series resistance where as G is the shunt conductance resulting in leakage current. This R is not related to this G, i.e., this G is not equal to this 1/R. Just to clarify with numerical values R is ,say about 0.3 ohm/km and G about, say, 5 micro S/km.

As high line current flows through R producing losses R is made zero for a lossless line.

As G is the shunt leakage conductance, there is a very small leakage current flowing to ground through G producing a very small loss. So G is also made zero ( or leakage resistance, Rg = 1/G , is made infinity) for a lossless .