I have two things to say up front: first, Gatewood already looks like a grown ass man, as a 6'5", 200 Lb, High School Senior. Second, he has, perhaps, the best power potential in this draft and, quite possibly, the most in the last couple of drafts. Now, we'll talk about the grown ass man build first. Gatewood adds a sense of irony to the name of the position he currently plays in High School as 6'5" is a somewhat absurd size for a SS (he'd be taller than Ripken, Jr. who was already an insanely large SS). That said, all scouting reports seem to indicate that at his current build he can handle the position and it isn't entirely out of the question that he could play SS in the pros (Ripken played the position every damned day well into his 30's). So, yes, he may stay there, but he's a fairly safe bet to play Third if/when he makes it to the Majors. To be honest, even the most glowing scouting reports on Gatewood indicate he'd be an average defender, at best, there, while he can be above-average at the hot corner (he also has well more than enough arm to play Right Field, as he hits the mid-upper 90's on the mound).
Now, about that power... Gatewood has a few things going for him, his size and strength, along with great to even elite bat speed allow him to launch balls further than most High Schoolers can dream of. The video below is from Steve Fiorindo and bullpen banter. It starts off with some Defense where Gatewood moves well, but that's not really the story here. Listen to his bat in BP. Solid contact, solid contact, solid contact, thwap, thwap, thwap. That just sounds wonderful. He's too noisy in his pre-swing routine during BP, wiggling the bat doing a front toe tap with a leg twist, then a long stride while loading his hands back. The risk he runs here is one of timing, but also one of consistency. Scouts note that Gatewood has very good bat control and that projects some to think he could end up with an elite or near elite hit tool. That said, many of the same reports also note that he occasionally messes himself up with all the movements and throws off his own timing and balance and he strugles against off-speed pitches because that long stride is built to destroy fastballs, but also to be perhaps comically off on changeups and breaking balls. He'll need to refine his mechanics as a pro. No one will want to sap his power, so I doubt anything gets dropped completely, but there's bound to be some efforts to lessen the bat wiggle and refine his toe tap/long stride so he can keep his power while still hitting for a respectable average.
Now, the in-game swing is a little less noisy than his BP swing, but watch the slow-mo at the end and see how he rocks back and forth a little before starting his swing. If the Pitcher alters his pace a bit, he could certainly catch Gatewood leaning the wrong way to start his swing. Scouting reports have noted that he does occasionally end up making contact when his back foot is off the ground and his lower half has already rotated, leading to weak contact (some of the same problems Larry Greene, Jr. has had, though Gatewood isn't doing that as consistently as Greene does). Gatewood's hitting coach is his father, Henry, who was a First Round pick of the Dodgers in 1982 (as a 6'2" Catcher). Jake certainly has the tools to be an elite hitter for both average and power.
Any team that drafts him will have to delicately balance those tools and find an approach at the plate that allows Gatewood to get the most from both. They aren't mutually exclusive and there are big power guys who also hit for high averages, but they're rare and it's more likely he'll have to sacrifice a little of both tools to maximize his overall game (essentially to generate power he'll need that big swing, which leaves him vulnerable to off speed pitches), the key for him will be both bat control to barrel up even when he's fooled and pitch recognition to know when to lay off a ball headed for the dirt or 3 inches off the plate where you can only make poor contact. That's the gamble here (as it is with many High School prospects). Gatewood could end up your cookie cutter AAAA guy who can mash, but Strikeout far too often to stick in the Majors. Of course Gatewood could also be Giancarlo Stanton, but as a Third Baseman.
It all comes down to his hit tool. I've seen it graded as anywhere from 35 to 50 at current on the 20-80 scouting scale (for those who don't know 50 is average) and I've seen projections anywhere from 50 to 70+. His power is a current 60 (at least) and projects to 70-80. His speed is going to end up being average, as will his defense, while his arm is going to be plus or near plus. I honestly see 2 outcomes from this draft in regards to Gatewood, in 20 years we either look back and say "My god, how did we pick X over Jacob Gatewood!" or "Who the heck is Jacob Gatewood?".
If you found yourself just dying to see (and hear) more of Gatewood's swing, here's another video (same source) that runs over 5 minutes with ~4 minutes of BP and game swings from darn near any angle you can fathom. What I like in this video is you can see more clearly he stands in an open stance, which he closes after the toe tap. His swing, at times, in BP does get a little video game, home run derby-ish, as he leans and uppercuts hard to try to launch balls. He takes some game swings just after the 4 minute mark and around the 4:30 mark you can see one of those awkward swings where he's way out ahead of the pitch and manages weak contact because he got caught rocking the wrong way when the pitcher wound up and he started his swing.
Whether the Phillies draft Gatewood says a lot about how fixable they think his swing is. If they take him, I would be very excited, because he's an elite talent that doesn't come available often, but I'd also be apprehensive due to the volatility to his profile (not personally, he seems like a good guy and does well in school, but his profile as a player).