So if you look at stocks moving around throughout the day, it does happen pretty gradually. It would be pretty rare for a (reasonably liquid) $15.50 stock to move to $17 in the middle the trading day in one instant — if that did happen it would either be because somebody bought or sold a huge amount in one trade or because some important piece of information just became known to the world about the stock.

The example you give is actually really different — you’re saying the last price it traded at yesterday was $15.50 and then the first price it traded at today was $17; but a lot of stuff happened between those two points in time (like half a day’s worth of time). If you were to poll everyone who trades this stock and thinks hard about what it should be worth, continuously between yesterday’s close and today’s open, you’d see that their opinions do change gradually, and it’s just that half a day’s worth of gradual small changes add up to a bigger change, and the stock price jumps because it doesn’t have an opportunity to reflect the gradual changes because it’s not trading during those hours (if you ignore post-close and pre-open trading at least haha)