It kind of seems like Gabriel has it out for Sam specifically in the first part of "Changing Channels". He throws Dean into his favorite show in the first "channel" (and we know Gabriel is not a fan of the show himself because he missed the crucial detail of Dr. Sexy's cowboy boots—he'd probably been watching Sam and Dean for a while and saw Dean in the motel obsessively watching the show). Yes, Dean gets shot in the back in the Dr. Sexy universe, but that's specific retaliation after Dean refuses to play his role with his "patient's" husband? Prior to that, Sam gets slapped in the face by Dr. Piccolo (and she TRIES to do it again). Hell—shooting Dean invokes Gabriel's previous torture of Sam in "Mystery Spot". Then in the next "channel", Sam gets hit in the balls and the two questions the gameshow host asks are negative about Sam. The first question is who did Sam choose over Dean (Ruby) and the second is asking Dean if his parents would be alive if Sam was never born. In the third "channel", Sam is forced to say he has genital herpes, while Dean just plays basketball in the background.
It's in the next, more neutral sitcom scene, where Dean pisses Gabriel off, that things (arguably) change. The next "channel" is a procedural cop show and Dean immediately rants about how much he hates procedural cop shows. The scene after that transforms Sam into Baby, which obviously sucks for Sam (Gabriel has NOT let him off the hook at all) but also: who messes with a man's wheels?????