It could be, it could also explain why Kei didnt remember Katou at all. Because if Kei was in gantz as a kid and meant Katou during that time, he would have forgotten all about him after being released. Also note that Kei doesnt seem to remember all the crazy shit that did as a kid either.
You sure you're reading Gantz? Kurono recognizes Katou before Katou recognizes Kurono.
Kurono recognizing Katou is pratically the first thing that happens in the manga...
And for the things he pulled off, yup, he pretty much remembers them alright, as Bobo already said.