The most suitable GCP database for migrating a 20 TB on-premises operational system transaction data is Cloud Spanner. Here’s why:

• Cloud Spanner: It's a globally distributed, scalable, and strongly consistent relational database service designed for mission-critical transactional workloads that require high availability and global scale. It can handle very large datasets, making it ideal for a 20 TB operational system.

• Cloud Bigtable: If the data can be modeled in a NoSQL wide column format and requires extremely high throughput, Bigtable is an excellent solution. However, for a traditional operational system being moved from on-premises, Cloud Spanner is likely the best choice.