How to deal with tanks as GP(or any other champ)

From what I see, I was doing well in the mid game. This was until the game came into the late area. How the tanks have too much armor/Mr to the point where I can't even do damage to them even with a last whisper and more pen. What can I do? From my most recent game, my team had many deaths which got their team far. We did well having great team fights until the tanks couldn't be killed. Also planning on selling my infinity edge and buy more pen, but I was too late. No one can peel for me since they engage hard and I get deleted by the cc and zed. So what can I really do in these situations? I also understand we had too much ad, but if that situation happens again what can I do?