I understand your point about not wanting to screw with a defense that is playing well, and also that the problems on offense go deeper than just WR, but as things sit, Amari Cooper has proven to have the ability to play at a top level in the NFL, and Randy Gregory has 2 sacks in 3 years. As for the team going through all the crap with Gregory, if trading Gregory could help bring in a high quality receiver then going through the crap wasn't in vain.
That said, Oakland isn't going to trade Cooper straight up for Gregory, and I don't even think they would trade Cooper for Gregory and a 3rd rounder like fred suggested. I just don't see that Gregory has any notable trade value at this point.