There is a part of Sam that is quite devious, under all the long-suffering earnestness, and that part pipes to life now. If Gabriel really can't leave, then Sam might as well take advantage of an opportunity.

'So,' he levels a look at Gabriel, still from his place against the wall, 'You say you're not just a dream, and I'll take your word on that for now, so. You got dragged here by the rift same as I did; you don't... know what's behind it, do you? What's-- wielding all that power, I mean. I can't believe it's just a random time-space... whatever acting without motive.'

Presuming Gabriel is real, that does at least somewhat soothe his fears about demonic powers, a repeat of the clusterfuck with Azazel's children. He is an archangel, after all; if demons were involved, presumably he would have smitten them into their composite atoms by now.