The value of any guitar is simply what a buyer is willing to pay.
I have a strong suspicion that George Gruhn pens very few appraisals that conclude with the phrase "What a buyer is willing to pay".
Yes & no.
For vintage guitars, "what the buyer is willing to pay" mostly only holds true in moving the price upwards; if I walk in a shop, see a guitar priced at $10k and say I'm only willing to pay $500, the price doesn't become $500.
But Gruhn (and other dealers) know and understand what is popular in the market and what has fallen out of favor (even if popular before). So Gruhn knows what th market as a whole is "willing to pay" and also how that might be different in the U.S. vs Japan, for instance.
What is a 1959 LP Standard worth? I played one once right after the owner came from Gruhn's. Tuners were replaced with Grovers (typical) and it had plenty of honest playing wear. What is the price for it today? Hard to say because lately sellers seem to not be posting that info (at least on the web), but you can bet north of $200k, and maybe very much more. In 2000, Gruhn appraised the guitar at $65k, because that was what the market would bear
at that time.
Is that Les Paul intrinsically worth more than a house? No, but there is a very small available supply of them and a lot of people with deep pockets around the world wanting to own one. That drives the price up & up whenever one is made available for sale.
So Gruhn says, "whatever the customer will pay" indirectly when he appraises. Especially in the cases of guitars/amps which once were hot commodities but the market has fallen on them and the prices dropped.