Did you just download the app? Or did you actually subscribe?
Go to the App Store app. Tap on your account name at the bottom left of the screen. On the next screen, tap on Account Settings in the upper right. Tap on Subscriptions. Look for ChatGPT. If it's there, tap on it, go to the bottom and press the Cancel Subscription button.
If ChatGPT doesn't appear in your App Store subscriptions, you may have subscribed directly through ChatGPT. Here are the directions one AI gives on how to cancel that type of subscription:
To cancel a ChatGPT subscription, log in at chatgpt.com, click your profile icon > Settings, then My Plan or Account, find the Manage option, and click Cancel subscription, confirming your choice to stop future payments after your current cycle ends.