Looks like you inadvertently purchased a subscription (with one month free) from the Tapatalk app. The gift probably refers to a gift to you, rather than the author of the post. I don’t know of any mechanism that would allow you to gift a subscription (or purchase) to another person without going through the App Store.
To cancel the subscription open the App Store, tap on your account icon at the top right, and choose Subscriptions. Select the Tapatalk app there, and Cancel Subscription.
If none of that seem to do the job, go to the Apple.com site, choose the Support menu at the top and follow the appropriate icons. App Store support is free, and they should be able to help you figure out what happened, and cancel as needed.
App Store support is free.
If you feel the placement of the link was misleading, you should send some feedback to Tapatalk. Keep in mind ads in ad supported apps are often hard to distinguish from content, and the developer seldom has complete control over how the ads look.
It does seem like Taptalk should be able to make their own ads obvious, but to give them the benefit of the doubt, a gentle reminder in the form of some feedback might be worth while.
