From their documentation
https://cloud.google.com/free/docs/gcp-free-tier (see the row for program initiation)
.....You are not charged unless you explicitly enable billing by upgrading your Cloud Billing account to a paid account. You can upgrade to a paid account at any time during the trial. After you have upgraded, you can still use any remaining credits (within the 90-day period)........
This means
1) If you are still within the 90 day free trial period or you have not exhausted the $300 credit, you will not be charged
2) If you have passed the 90 day free trial period or you have exhausted the $300 credit, then you will be charged going forward. But remember, the charge only happens after you have exhausted the free daily quota
3) If bullet 2 has happened and you want to prevent being charged at all (i.e. no charge even after exhausting your free quota), the only way I know (and I might be wrong here) is for you to delete your billing account. However, most services require you to have a billing account. For example, you can't deploy your app to production without having a billing account