I also beg to differ. Stamps of this period and earlier are often valued at about half to a third of the normal mint value, with all other things like centering and damage considered. See the catalog prices given for mint classics with no gum.
If stamps are really well stuck together, there is little that can be done to save the gum. Otherwise, there are various ways like freezing, steaming, using a sweatbox, mentioned here:
https://goscf.com/t/65867 but detailed in earlier threads. If the gum ends up very disturbed, the price is still going to be towards the no gum price.