The reports I have read on Gasser was that he wasn't high on the prospect charts largely because of a FB that topped out around 90 MPH. During the 2020 Covid lockdown, he committed himself to working out and that added velocity and tightened up his slider. He now throws 93 regularly and can hit the mid-90's, and he has a "plus" slider, with a change-up that could turn into a plus pitch.
This year, he has thrown 90.1 innings, and has a 115:28 K:BB ratio (4.11 K's per every BB). I've always like this ratio for pitchers, as K's are really helpful to pitchers, while walks are really harmful. Kelly has thrown 91 innings, with a 119:52 ratio (about 2.29 K's per BB). For better or worse, the Brewers seem to be looking for hitters that make contact, play good defense, and have good eyes at the plate, and pitchers with good command and the ability to strike guys out. Melvin liked hitters with one "plus, plus" tool (usually power), and pitchers with big arms with questionable command, Stearns does not. Kelly would probably have been more of a "Melvin guy," trying to harness the big arm and fix the command. Gasser is more of a "Stearns guy," already having the command and polishing up his game. It doesn't make one good and one bad, but Stearns is the guy pulling the strings these days.
They are both prospects, so their is risk to both. I don't follow prospects closely, so I have to rely on what reports say, and yes, I have to have faith in the Brewers. Stearns called Gasser something to the effect of "the best pitching prospect in baseball," so that was kind of surprising. This goes beyond simply talking a guy up. Stearns inherited a franchise that looked doomed to a long rebuild process, and quickly turned it into a playoff contender, and he has been slowly turning a bad farm into a good one. It looks like he feels Gasser has a better shot at helping the Brewers at the MLB level than Kelly does.