Said gym memberships tend to be because you signed an annual contract, with specific requirements to cancel early.
Using one of these one-time-use cards won't get you out of the debt itself, and these sorts of gyms will happily wreck your credit by sending it to collections.
I've heard of some cases where the contract auto-renews and requires an in-person presence during a limited time window to cancel when they're quite busy.
While I'm sure these places have it in their terms, just making someone sign a contract to agree to it doesn't make it not-unethical or not worth criticising. It just makes it "not illegal in some jurisdictions"
Using one of these one-time-use cards won't get you out of the debt itself, and these sorts of gyms will happily wreck your credit by sending it to collections.