SubGPT is a free tool that uses BingGPT to find more subdomains based on a list of already discovered subdomains for a specific domain. The tool can be installed using pip or by cloning the GitHub repository and running the setup.py file. To use SubGPT, a Bing cookie is required, which can be obtained by installing a cookie editor extension, logging in to Bing.com", and copying the cookie. The standard way to run SubGPT is by specifying the input file, output file, and path to the cookies.json file. If no output file is specified, the output will be shown in the terminal. SubGPT can also be used to generate subdomains without resolving them by using the --dont-resolve option. The tool is designed to be used after discovering some subdomains using other methods, and it's important to ensure that the subdomains list only contains subdomains from one domain. SubGPT looks at A/CNAME records to determine whether a subdomain exists and can detect wildcard on first-level subdomains, handling it automatically.