Pierre just trained for 19 months harder than any fighter does. Try restoring your strength to a professional level after an ACL tear to comeback and fight arguably the best fighter in your division while you were out
The layoff doesn't matter. The only time GSP ever entertained the thought of fighting Silva was after the UFC 112 fiasco and after the first Sonnen fight.
Ever since the Belfort fight he's gradually changed his opinion on fighting Silva. He made excuses about needing to talk to his sponsors first

, and how he doesn't want to move up in weight even though, IIRC, it's always been rumored as a catchweight fight.
I mean, Dana and Anderson are on board, it's all on GSP now. I hope Dana is able to convince him, but from what I've seen over the last year, I really doubt the fight will ever happen.