I would say it does absolutely affect the value if it is a re-gummed ... counterfeit for that matter because either one is recognized in retail catalogs.
In the Scott Specialized 2021 catalog, there is no reference or pricing of gum with regards the the listed postal counterfeit prices in that section of the catalog. There is some gum discussion but it is for the subject matter of distinguishing the counterfeit from the genuine issued stamp. And, to repeat what was said by another before, the counterfeit C-15 has a higher catalog value than the genuine examples.
I suggest the creation new subject thread be created and named Fakes, Forgeries and Counterfeits
If you put "forgeries" into the search function offered in SCF and use the Google box, you will find near 100 listings. In one it is pointed out there is a 14 volume set of books (approx. $400) just on forgeries Germany. You will also find that the topics seem to be country, issue or forger specific.