I don't know anything about GP but if the product quality is there as you say it is, then this is imho all a brand marketing problem....
You mentioned a lot of potential factors but I think these all roll up into poor brand marketing. Luxury brands for anything, bags, watches, coffee machines, once the base level of product quality is there to play with the big boys, everything else is brand marketing. Heck, even if the quality isn't there, you can fake it until you make it if the brand is strong enough. All those watch makers at GP are being let down by poor brand management.
With a strong brand comes demand, with demand you can begin to figure out resale, supply, auction results, manage supply and distribution, etc. but without that the brand is powerless. If a brand is strong, heritage can be a useful marketing tool to make it stronger, a moat, but if the brand isn't strong heritage can just mean old.
I don't really know anything about GP or have an opinion about it and that's exemplary of how bad the marketing is. By bad I mean non-existent. I'd like to think I'm a little more interested in watches than the average guy on the street and yet I have no real knowledge about GP and no-opinion about them good or bad. Part of that is my own lack of interest but part of that is GP not doing what they should be doing to put the brand and products on my radar. Who is the CEO of GP? Is he or she out there working the media? Brand ambassadors? I don't know what the brand stands for or what they're good at and GP should figure out how to fix that. GF on the other hand is very clear about what they stand for and what they're good at, so clear they write it on the watches :P
As for the watch in question, any brand is going to be challenged to sell steel watches at $10k AND gold complications at $320k. JLC marketing kinda sucks too but they've done enough that they can manage that range and wield their heritage in a way they can drop both a steel Reverso and $1MM Westminster and both feel part of the brand and people trust buying both those level watches from JLC. That requires a lot of trust in the brand. Other brands haven't been able to build that trust and had to split the brand (e.g. Seiko / GS / Credor or Scion / Toyota / Lexus) and trust is something beyond just brand positioning, awareness, and outreach, it's a strategic part of the brand marketing that needs to be thoughtfully implemented over years of campaigns and intertwined with heritage to create something foundational.