You can cancel your G-Plans account easily by logging into G-Plans.com and navigating to your Settings page.
On the Settings page there will be a section titled "Your Membership." Here you will see your membership level and a button to "Cancel Account."
From there you will follow the prompts to cancel your account.
If you purchased your account through either the Apple App store or the Google Play store directly, you will need to cancel your account through your respective store. Please see the following help articles:
Apple App Store Membership Cancellation:
Google Play Store Membership Cancellation: