As per the Google's doc
New projects and Free Trial accounts do not receive GPU quota by default.
You must have GPU quota before you can create instances with GPUs.
All free tiers users are initiated with 0 GPU, which they will have to submit a request to increase the quota. However, to file a request, one has to upgrade his account to a paid account.
10 minutes ago, I am still in the free tier and all my projects will be billed for the $300 free trial promotion credits.
I had to upgrade my account to submit a request for GPUs. A minute later my request was approved. When I try to create an instance again, I am charged according to the price sheet.
Updates
I have just reached out to a Google Cloud Support member and here's the result.
After an account has been upgraded, the unconsumed promotion $300 credits will be consumed first. Once the credits have been consumed, the charges will automatically then charged to your payment method on file. Since you have upgraded your billing account, you will not be notified that your free trial credits are consumed and that charges will be taken on your credit card. However, you can actually monitor your charges on your account by simply logging in to your console and check usage in the billing section.
In short, you may upgrade your account first and train your machine learning model and monitor the charges in the meantime to avoid unintended charges.
Meanwhile, Google has just released out the Colab sometime earlier to the general public with a noble goal of dissemination of machine learning education and research.
You can use GPU as a backend for free for 12 hours at a time.
The GPU used in the backend is K80(at this moment).
The 12-hour limit is for a continuous assignment of VM. It means we can use GPU compute even after the end of 12 hours by connecting to a different VM.
This essentially means you can train your ML model for free! Hurray!
Hope my findings help.