Code Geass has an interesting plot (in fact, it´s the only reason i watch it, since i really don´t like mechas), Gurren Laggan is just a bucn of cliches bunched together with a (plain and simple) linear "argument".
Reason why TTGL is popular? Kamina dies
Reason why Geass is popular? Nice plot, nice animation, good char development, popular seiyuus (i´m not into that, but seems Jun Fukuyama= instant succes =/), etc...