Gilbert does his job well. He's a good 4-5 guy, and is paid like one. He's not the greatet fit for Therrien's "system", but he blocks shots, moves the puck and plays generally sound positional hockey. Needs to be paired with another fleet of foot defenceman though.
The only two players you name dropped who Emelin is arguably better than is Gorges, Mitchell and Carle. And they aren't exactly playing even average hockey right now. Everyone else is clearly better than Emelin, from the eye test to advanced stats.
Regehr isn't even a for sure top-4 D in LA right now. None of your estimations of where he'd line up on other teams is accurate either. He's not Top pairing in Detroit? Have you seen Dekeyser and Smith play. He's on par with Ericsson. Ask Detroit fans what they think of him. And Nashville? Have you seen Ellis, Ekholm and Josi this year? That's excluding Jones. He'd be number 6 on that team, just ahead of Volchenkov.
Emelin is a valuable defenseman, but He's not a top 4 guy, unless he's paired with someone that can make up for his many blunders and lack of puck movement. He should not have the 2nd highest ES TOI.