every second GT game in the generation sells less than its predecessor.
So no, GT6 is not a flop.
While you are right in the aspect of predecessor sales, no GT games has been forcasted to only move 50% of the previous game. The lowest (if we go by VGC #'s ) was from GT3- GT4 which was still 78% of it's predecessors sales.
" Rebellion Against Tyrants Is Obedience To God"