Young top 4 d with good offensive upside and maybe like a second or third round pick.

This is a tough one. I agree with @Philly_fan1 . Noah Hanifin comes to mind. Guentzel is a winner. He's produced away from the HOF'ers too. Critics will say that the HOF'ers still had to face the shut down lines, but Jake's playoffs have been GREAT to watch. Is Hanifin about what you're thinking?

I’m honestly not sure. I’ve ranged from:

A first rounder
A top 4 dman and a second
A top 9 forward and a higher end prospect
Two prospects and a late pick

I think looking at picks and prospects is the wrong way to look at his value. He's too young of a player to be trading for picks and prospects.
Why trade a 23 year old who plays well for a 20 year old unknown and some draft picks. That seems pointless.
On top of it, Jake really has two values. There is regular season Jake and there is playoff Jake. One is a fairly good player, the other is sign him now.
If I were to trade him, I would want someone like the breadman in return. Fairly young, speedy and is a proven performer. He's definitely worth more than a 2nd line pairing player. I know I wouldn't trade him for a lot of teams 2nd line defense men.

His value as a point producer and winger for Sid far outstrips anything the Pens could reasonably get back for him in a trade.

Hanifin isn't bad comparison for value but young top 4 dmen tend to have more value than forwards but definitely in that ball park.
It would have to be an offer the Pens would be too stupid to not take for Guentzel to get moved. Like a package that gets you Hanifin and Lindholm from CAR. Now that isn't realistic for guentzel alone but that kind of package that would be acceptable. a top 4 dman and a young talented top 9 wing. Probably won't get that from a team but that is what it would take for Pens to move him as he is too important.