Depends on the buyer, what the signature says (ie, dedicated to you or just his name,) whether you can prove it is Gary's autograph (he is well-known to have signed numerous things so its very likely signed, but still evidence
never hurts) and so on and so forth...
So people, the really hardcore guys, probably won't be interested -- the book is incomplete, and "damaged" with the writing.
To others, it can be a real wildcard... so, I'd say a flux of +/- 20-30%, but possibly more on the open market where eBay gets the strangest bids a-flying.
Me, I'd not even consider it. Selling, that is. Buy-wise, since I don't have a true 1st print
PHB, I dunno... have to see a picture and then compare to others.
Those who can, don't. Those who should not, do.