They both strike out a ton and both hit for power. However, Wallner is pretty much the anti-Gallo, when it comes to BABIP, which is what this article highlighted for him.
BABIP is a very interesting part of a player's portfolio. It's often used as a synonym for "luck", in that the league as a whole typically hits .300 on balls in play, and if you don't know anything else about a player then if his BABIP is very far from .300 in a given season you might expect the so-called regression to mean.
However, not every player's own baseline BABIP is .300. Joe Mauer kept his around .340 for his entire career. Luis Arraez is on a similar pace, so far. I don't believe in "luck" very often, not in a game where trained athletes are competing against other trained athletes who are all trying their best on every play. BABIP regressing to .300 is a useful guide but is not some sort of law of nature.
Conversely, a high BABIP is not a Hall of Fame kind of profile, by itself.
If you take a look at hitters who've had 2000 PA in the past 4 seasons, and whose BABIP in that timespan has been .330 or higher, you have some absolute studs like Aaron Judge, Bryce Harper and Freddie Freeman, but you also have a sprinkling of "wait, him?" players like Amed Rosario. If a hitter has a high BABIP and also walks a lot and hits home runs (or lots of doubles) then that's a potent combination, but some guys have BABIP galore and don't back it up with additional value. It could correlate, but it's nothing close to a cause-effect.
Low BABIP is by itself nothing to hope for, but at the low end of the same group of players*, there is a smattering of guys like Max Muncy, Pete Alonzo, and Kyle Schwarber who are highly productive despite BABIP in the .240-.260 range. Our own Carlos Santana brings up the rear in this group with a .237 BABIP, and yet he's done enough on top of that particular facet of his game to earn major league contracts and will get another one next season.
So... what I'm leading to here is that Wallner's career BABIP is currently .359. That's across merely 3 partial seasons, so there is plenty of room for doubt, but all three partial seasons have been in keeping with this, rather than drastically up and down. Joey Gallo? For his career, his BABIP is .254. A full 100 points lower than Wallner.
The two batters share some common ground, but in this one area they could hardly be more different, and I think it's an under-studied aspect of analytics. There are many ways to carve out a successful major league career, but BABIP by itself, once it stabilizes, doesn't tell me enough. Gallo has fewer PA than Amed Rosario, yet has scored more runs and driven in more runs, so in a vacuum you want to know more about Gallo than just his low BABIP (namely that he supplements his BA with walks, and he hits balls over the fence).
If Wallner continues to walk and hit homers, his BABIP suggests he could be a lot better than Gallo, at the end of his career. If the BABIP isn't entirely a mirage, of course. That's the big unknown touched on by this article.
* I'm quick to point out that this choice of players contains a statistical bias built in: you have to be pretty darn good in the first place to amass 2000 PA in 4 seasons. But then again, do I CARE about the batting profile of bad players?