GuardRails is described as 'continuously scans your GitHub & GitLab repositories to alert you of any vulnerabilities and security issues. Get started in minutes'. There are four alternatives to GuardRails for a variety of platforms, including Online / Web-based, SaaS, Microsoft Visual Studio, Azure DevOps and Windows. The best alternative is Snyk, which is free. Other great apps like GuardRails are Backhub.co (Freemium), lgtm.com (Free) and WhiteSource Bolt (Free).
lgtm.com is a platform for code analytics. It's free to use for open source software; results for over 55k projects are readily available (add your own!), has automatic code review pull request integration (GitHub, Bitbucket, GitLab), and much more.
WhiteSource Bolt is a FREE tool that scans all of your projects to detect vulnerable open source components. It provides actionable remediation paths to enable a quick fix. Available on Github & Azure DevOps Marketplaces: bolt.whitesourcesoftware.com.