More soul destroying than "your performance is shit, so shit I see no hope your worthwhile, get out?"
I think the GP is overly optimistic about a PIP. In general I'll say when I've put someone on a PIP, it's been with the expectation of firing at the end. "We have discussed this problem and I've made it clear you need to improve, this is your last chance, I need to see x, y, and z (where X Y and Z are as concrete and measurable as possible) or mm/dd will be your last day". It's making clear that whatever issues are involved have come to a head, and this is the final chance to fix them. My general assumption is that if spelling out the issues and providing coaching hasn't resolved the issue the PIP probably won't either, but I do see value in a clear process vs the cut just being whenever I decide to do it, with no warning that day is coming.
I have also actually seen this work in practice. I've had people who multiple conversations and coaching seemed to make no inroads, but putting that clear "if this isn't resolved by X day, we're done" expectation out there seemed to make it "real" and they've completely turned things around. I have promoted people I've previously put on a PIP.
That said my advice remains, if your put on a PIP it's time to start looking. I think many (most?) managers d use them cynically where a PIP is more paper trail than final warning, and the employee may be getting fired regardless. Even if that's not the case you've just been told very clearly it's not working. Something isn't working and it's better to hedge your bets and look for another job that fits better while trying to improve.