Is this just a bad matchup for Gobert? He's been invisible all series
I don't think this is a series where Gobert can perform near the top of his game. So in that respect, yes a bad a matchup. He is forced to spend too much time out on the perimeter defending LAL's 3 point shooting forwards and switching onto their dominant shot-creators. 
That limits the value of Gobert's interior defense and defensive rebounding because he is spending all his time out at the three point line. Those are his best strengths and they are being curtailed. So yes, bad matchup. 
On the positive side, Gobert is still playing good defense on the perimeter. It is a lie that he is a bad switch defender. He is a good switch defender. LeBron doesn't like attacking any of Minnesota's bigs one on one off the bounce. Luka gets some joy but it is more of a mixed bag. 
Minny's defensive numbers with Gobert on the floor show he is still having a positive impact which is what you would expect given his track record of being a good switch defender / good perimeter defender. 
It is just not as large of an impact as he can have defensively when he is allowed roam around the paint and muck up the other team's offense. It is a positive impact but smaller. 
So look for good play rather than great play from Gobert in this series. And he is delivering that. 
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Most of his low scoring totals are failures of his teammates to find him when he is open or has an advanageous matchup inside. This has been issue for Minny for 2 seasons now. The only guy on their team who finds Gobert consistently is Mike Conley and Conley is now a shadow of himself. Without Conley feeding him, Gobert gets fewer attempts at the rim. On a different team, with proper playmakers, Gobert would be scoring 10-14ppg in this series. It is a teammate problem. Not a Gobert problem. It would be the same with any other rim runner / rim finisher like Lively or Capela if they were put on this Minnesota team. It is their lack of passing that is the issue.