I'm a Raiders fan, and I think the potential reward doesn't outweigh the risk.
Even if Gabriel starts all year, I wonder what kind of consistency he'll have. Now, I could be underrating him. He came in very raw, and despite some drops and quiet games, he has developed nicely, if not as quickly as Raider fans would like. He has the speed and size, but don't forget, if Curry is right and Porter pulls his head from his backside, Gabe could be the #4 WR quickly.
Now, those are both big questions, and I expect bigger numbers from Gabe than either of them, but a shaky #2 on the Raiders isn't really a guy I'd be targeting.