But couldn't B say he accepts the license for some copies, but not for others? For the copies that A gave him, he can reject the license and distribute them by copyright law (first sale). For anything that he wants to copy or modify himself, he has to accept the GPL, but he can accept the GPL for just those particular items.
I am not a lawer but I don't think it is possible. In particular read section 4 of the GPL. It seems that either you agree to the GPL and you are bound to it. Either you do not agree and the GPL do not apply to you anymore. I don't think you can agree to the GPL just for a limited copies of the software.
Olive