Synaptic is not available for Mac but there are some alternatives that runs on macOS with similar functionality. The best Mac alternative is Homebrew, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Synaptic and six of them are available for Mac so hopefully you can find a suitable replacement. Other interesting Mac alternatives to Synaptic are Nix Package Manager (Free, Open Source), Mas CLI (Free, Open Source), pkgsrc (Free, Open Source) and Pallet (Free, Open Source).
pkgsrc is a framework for building over 17,000 open source software packages. It is the native package manager on SmartOS, NetBSD, and Minix, and is portable across 23 different operating systems. Use one package manager across all of your systems!
PortAuthority provides a GUI for the MacPorts Unix software management system for Mac OS X. MacPorts is a powerful command-line tool for installing and updating a variety of Unix-based applications for the Mac.