I think they told Gardner first out of respect. They will try to keep him on the PS if think he has any upside potential. He seems to have physical limitations, so they may be ready to let him go all together.
Most people don't seem to get this. Good head coaches like Garrett (and probably most NFL coaches) care about the person sitting across from them when that player has all the right stuff. They actually hope they have success elsewhere when they know they're good enough. Coaches will even call other coaches and say "we don't have room for this guy, but he could probably help you." Coaches usually aren't as cut-throat as people sitting in their underwear typing on a computer.
Sure, they'd love to get Gardner on the PS. But like others said, the Cowboys probably realize he's good enough to play for somebody, and they actually want to see him have success in this league, success fulfilling his dreams. It's a gamble, but they keep the ones they think will be the best at their position, sign the good ones who may be too raw to help anybody right now to the PS, and wish the others well.