The correct answer is Cloud Monitoring. This GCP service is tailored for managing and analyzing time-series data, offering capabilities to collect, visualize, and alert on metrics and logs from your GCP resources and applications. It provides a comprehensive overview of your system's performance and availability, focusing on metrics like CPU utilization, latency, and error rates.

• Cloud Dataflow is utilized for executing batch and streaming data processing pipelines. Although it can handle time-series data, it's not specialized for this purpose.
• Cloud Firestore is a NoSQL document database that supports real-time synchronization and offline capabilities for mobile and web applications, not specifically for time-series data analysis.
• Cloud Spanner is a globally distributed relational database service that can store time-series data but lacks the specialized features for its analysis that Cloud Monitoring provides.

Thus, Cloud Monitoring stands out as the GCP service dedicated to managing and analyzing time-series data.