GCLI2API is a production-grade API proxy that converts Google Gemini CLI and Antigravity credentials into OpenAI, Gemini, and Claude compatible API endpoints. It manages OAuth credentials with automatic rotation, error-based cooldowns, and load balancing, providing a unified interface for multiple backend services. The system includes a web-based control panel for credential management, real-time logs, configuration editing, and batch operations. The architecture is a FastAPI web application with modular routers for each API format (OpenAI, Gemini, Claude) and two backend modes (geminicli, antigravity). Format converters handle bidirectional translation between Anthropic, OpenAI, and Gemini schemas, including tool calls, streaming, and thinking blocks. Storage is abstracted via an adapter pattern supporting SQLite, MongoDB, and PostgreSQL, with optional Redis caching for credential pools. Key features include streaming support with fake streaming fallback, anti-truncation via synthetic tool calls, thought signature preservation across format conversions, and a keepalive service. The system relies on httpx for async HTTP, Pydantic for data models, and external Google Cloud APIs for OAuth and quota management. Deployment is supported via Docker, docker-compose, and platform-specific install scripts for Linux, macOS, Windows, and Termux. This is a pure application with no data research or ML components. It demonstrates strong engineering practices in credential management, format conversion, and async I/O, though it suffers from some code duplication between backend modes and limited test coverage.