What is PSA’s rationale for needing the gum stuck in place? That sounds insane to me. First, a loose, moving piece of gum some 40+ years later only increases the possibility, though does not guarantee, that the adjacent card was never adhered to or otherwise damaged. So they want you to MAKE the gum stick by exposing the pack to intense heat? If not melting the wax coating outright, at the very least it will damage or alter the wrapper and/or stickers into something less certifiable than what it is now. And making the gum stick, even if that were the only result, to me only increases the likelihood of damage to that first adjacent card.
I’d take my chances and submit as is. At least contact them and ask. If they return and refuse to grade, they’re idiots.
And this is why people pay a premium for items graded by 3rd party grading services?