Option 1: Pay for Copilot Enterprise

This is the Knowledge Base feature for Copilot Enterprise, where you will set up a group of repositories and for every prompting it will go there bases RAG, read more here https://docs.github.com/en/enterprise-cloud@latest/copilot/customizing-copilot/managing-copilot-knowledge-bases

Option 2: MCP Server

Create your own MCP Server reference to any source, even GitLab, GitHub, Slack, Database and so on. This technique is quite new but you can try to setup any sources you want, for more details here https://github.com/modelcontextprotocol/servers