Personally, I think the quirk is OP, even if it's 'flaw' seems balanced, it doesn't happen until hours later, and doesn't effect the quirk during use, he even says it's not bound by physical restrictions.  Basically, he can change gravity within 300 ft however he wishes, with pretty much no limitations, with the only downside being it happens to him later on at midnight.  To me, everything about it screams OP, because as an example, we could have like the end of Season 1, where villains have attacked the school.  Well, he can simply put a stop to it by increasing gravity for all the villains in a 300 ft radius, stopping them in their tracks, while everyone else cuffs them and takes them away, with the only downside being that that night, he has increased gravity pinning him down for a bit.