Systems like DRS and OAA aren't perfect, but unlike your small sample of games eye test actually evaluate every defensive play a fielder makes and assigns plus or minus grades based on likely outcomes with regards to relative difficulty of these plays. Giving out gold gloves based on the eye test is simply not something that would work. Who has time to watch every single inning for every player in MLB in order to hand out these awards?
On the same note just because you watched a handful of games where players misplayed balls this does not somehow create a representative sample for an entire season of play. I recall Gurriel made some really nice plays against the Blue Jays in the series against the Dbacks in Toronto this season, so if I only watched those games I'd have the impression he's a great defender. Conversely Matt Chapman has had a bit of a rough month defensively since coming back from the injured list. If I were to have only watched the last few weeks of Matt Chapman innings at third base I could easily come to the conclusion that he's a bad defender at third base, but this would be far from a fair determination to make.
Gurriel is a player who had a relatively late start in the outfield, and to put it mildly it was far from a smooth transition. He's improved leaps and bounds as a defender in the outfield from the ultra shaky early days when he was still learning the position. He was a legitimately terrible defender starting out and more often than not resembled a deer in headlights when fly balls were hit his way, but he's slowly turned himself into a solid defender in left field. He's been more favorably viewed by the DRS system but a big part of that was due to OAA not factoring in throwing contributions which is his primary strength in the outfield. Once you factor in throwing Gurriel now rates as an above average defender when factoring in the total defensive contributions. On the Kiermaier comparison Gurriel doesn't rate close to as highly as Kiermaier as Gurriel is credited with 11 DRS vs Kiermaier who is credited with 17 DRS.