I keep getting billed for traffic but all of my servers are offline.

GP2 are EBS or Elastic Block Storage. EBS storage is the storage used by EC2 instances when they are running. You probably have a disconnected storage volume. If you go into your account into the EC2 service on the left panel in the Elastic Block Store section, click on Volumes. You probably have a storage volume there still. While you are there, you should also ensure you don't have any snapshots either. These would be backups that can also cost you money. Remember to also make sure you are viewing the region that the costs are showing up in your bill.

This is not traffic charge, but storage charge for EBS volume. Your instances didn't have Delete on Termination option enabled when they were terminated. Unused and overlooked Amazon EBS volumes contribute to AWS costs. The lifecycle of EBS volumes can be independent of Amazon EC2 compute instances. Therefore, even if the EC2 instance associated with the EBS volumes is terminated, the EBS volumes tend to persist unless you select the Delete on Termination option at launch. Also, instances spun up and down as part of development and testing cycles may leave orphaned EBS volumes if there are no workflows in use to delete them automatically. These orphaned EBS volumes accrue charges while unattached. You need to look for unattached EBS volumes in Elastic Block Storage -> Volumes under EC2 console as there is a column named Attached Instances. In addition to unattached EBS volumes, unassociated EIP can also accrue charges. Please referred to https://repost.aws/knowledge-center/ec2-billing-terminated for more detail.