Gabriel doesn't tell Sam, but he knows what the so-called anti-possession tattoo really is. He knew, the first time he saw and all it took all of his tight angelic control not to betray his face. Sam's amused and amazing laughter is fading in his ears and the sunlight on Singer's front-yard seems to grey out, when Gabriel watches the hunter remove his shirt. Now he wished he never had let that bucket pill it's contents.
Because the tattoo over Sam's heart is not a normal one.
Yes, it's a anti-possession tattoo. Yes, it does ward against demons.
But also against everything else in this world.
Sam will never moan yes, yours under Gabriel, just as he can never growl mine.
The star symbol states it, Sam belong to himself and the surrounding flames are a silent claim. A claim from someone, who's rights are much more justified than Gabriel's. Who's rights are much older than Gabriel's. The archangel swallows. He could have loved Sam, but he can't when the Winchester belongs to already to his older brother.
There's only one being, who has the right to enter this body.
There is only Lucifer for Sam.
Gabriel curses fate and flutters away, with the intention to warn Castiel off his little Dean Winchester infatuation, because what is true for Sam and Lucifer is true for Dean and Michael as well.
Sam Winchester is left behind and never learns, what he could have had.