Why is it legal, does the gym need those 100 contractors to know my data for it to work? What are those for 100 different accountants? How did gyms or other businesses worked before the internet, did a guy walked to 100 different locations with papers in hand so those "partners" take a quick look?
If they want to send you a letter, they have to give your data to the postal service. Again, no consent needed.
This is legal because our whole economy is based on devision of labor. Privacy laws account for that.
My problem is with the 100+ partners that are OBVIOUSLy not partners and not required to have my data.
False equivalence, no online stalking company actually works like that (that would require a server-side hook). They all make the visitor go to the third party's desk and increase the tally themselves (via http request), giving the tracker company access to all the contact details of the visitor.
Before the internet, the owner took a shoe box of receipts to their bookkeeper every month. Those receipts had your name, date, etc. on them.
How, when I buy stuff in real world and pay with cash I don't ask for an Id Card, so why do you think the store needs names on the receipts? Is this something that happens in your country? For buying cars,land you need an Id, if I buy even an expensive electronics no Id is needed I just return the product and the receipt that has no name on it back.
I remember when my grandfather was doing accounting for a bar before Internet days, they papers were about the stuff not about people, like how many bear was bought, how much was sold stuff like that.
(By the way, a gym can and usually does share contract data including personal information with numerous third-parties such as external bookkeepers. This is legal under the GDPR without explicit consent.)