OpenSSL is a framework which implements the secure socket layer. It ships with a program/library which gives you access to all those functions.

GnuTLS is not available for Mac but there are some alternatives that runs on macOS with similar functionality. The best Mac alternative is OpenSSL, which is both free and Open Source. If that doesn't suit you, our users have ranked eight alternatives to GnuTLS and seven of them are available for Mac so hopefully you can find a suitable replacement. Other interesting Mac alternatives to GnuTLS are LibreSSL, mbed TLS, Themis and Network Security Services.
OpenSSL is a framework which implements the secure socket layer. It ships with a program/library which gives you access to all those functions.

LibreSSL is a version of the TLS/crypto stack forked from OpenSSL in 2014, with goals of modernizing the codebase, improving security, and applying best practice development processes.
Mbed TLS (formerly known as PolarSSL) makes it trivially easy for developers to include cryptographic and SSL/TLS capabilities in their (embedded) products, facilitating this functionality with a minimal coding footprint.
Network Security Services (NSS) comprises a set of libraries designed to support cross-platform development of security-enabled client and server applications with an option support for hardware SSL acceleration on the server side and hardware smart cards on the client side.
Software components for Data Protection, Secure Storage, and Secure Transfer. Trusted in high security, mission-critical applications for decades.
wolfSSL (formerly CyaSSL) is a small, fast, portable implementation of TLS/SSL for embedded devices to the cloud. http://www.wolfssl.com.