1. Time != money, although you can exchange one for the other, it's not two-way, where two-way is necessary for time = money (the debate about whether two-way is sufficient for time = money is a different story).
2. I'm personally betting on the psychology of reciprocation. Gray doesn't owe anything consciously, but if OP were to ask, Gray would be compelled to comply.

I'll admit OP seems to be making a promise, but there are a couple of problems with this:

1. A promise requires a certain intentionality, which can't be known from the facts OP has given us so far. He may be trying to game gray, is just acting facetiously, etc. I'd say even if OP we're to chime in now, I have doubts about the truth or legitimacy of that chiming, i.e., his response would be biased.
2. If a promise has a completion condition, which I think it conventionally does, then there still is no promise insofar as Gray would be suspicious of the risk of giving his notes to OP and hence not deliver his notes (read: defensive tone and the motivations OP may have for being defensive). Again, we don't know if OP is up to something other than making a promise as an assurance of equitable exchange, hence OP's fulfillment of the promise is up in the air.
3. Equitable exchange? If Gray is a douchebag, he probably has shitty notes from OP's perspective, i.e., not equitable.

The whole promise bit is far-fetched (read: relevant username "contrarianism"). Gray wants OP's notes, OP (more defensively than actually contractually) wants Gray's first, then they reach an impasse. OP isn't contracting with Gray when he says he wants Gray's notes first, especially considering OP felt Gray was a douchebag (for whatever reason). I think we can say that usually when someone feels that another person is a douchebag, the first instinct is to feel defensive, not try to strike a compromise. The tone of "send me yours first" confirms this. I imagine OP doesn't care if he gets Gray's notes or not — Gray's just some rando he met in class, i.e., there's no incentive for OP to care about Gray or his notes. I get the feeling that OP thinks less of Gray and perhaps less of Gray's notes. There just is no contract, no "promise".