gapicgen contains three binaries:
cloud.google.com/go/internal/gapicgen/cmd/genlocal: Generates genproto+gapics locally. Intended to be run by humans - for example, when testing new changes, or adding a new gapic, or generating from googleapis-private.
cloud.google.com/go/internal/gapicgen/cmd/genbot: Generates genproto+gapics locally, and creates CLs/PRs for them and assigns to the appropriate folks. Intended to be run periodically as a bot, but humans can use it too.
cloud.google.com/go/internal/gapicgen/cmd/genmgr: Checks for an outstanding gapic regen CL that needs to have reviewers added and go.mod update, and then does so. Intended to be run periodically as a bot, but humans can use it too.
See the README.md in each folder for more specific instructions.