Here you need to increase the quota according to your requirement and for editing quota you can get a guideline from this document. Please keep in mind that for changing quotas, you must have serviceusage.quotas.update permission. This permission is included by default for the following predefined roles: Owner, Editor, and Quota Administrator.
Also there is some more information about GPUs quota here, which might be helpful.You can also take a look at more details about Working with Quotas.
Though I am not sure if you are using Google Cloud Free Tier and trying to add GPU. Just for information, according to program coverage of free trial, we can not add GPUs to VM instances for Google Cloud Free Tier.