Cancelled stamps that appear to have original gum????

As zipper noted, those are "Cancelled to Order" stamps (CTOs).  Postal agencies in various locales print millions of stamps for the philatelic market, but realizing collectors are not going to pay the face value, and unwilling to discount mint stamps which could be used for postage in that locality, they cancel sheets of stamps (cancels are often printed on the sheets) and sell the sheets to the collector market at deeply discounted prices.

There are of course collectors who actually look for certain CTO's like the early Victoria stamps that all have "Melbourne" as the city with different dates.
There are also PTO's or favour cancels. Some but not all post offices allowed clerks to cancel stamps on envelopes never destined to go in the mail. Such covers can then have good value but generally lack other postal markings. Unfortunately, singles soaked off are indistinguishable from the real thing.
As noted, modern CTO's have little or no value.

stainlessb,

the CTO stamps are generally treated as postally used for both description and pricing. Most experienced collectors will recognize
the CTO stamps without it being pointed out to them.

Regards from Canada
Bill