If you’re using homebrew, you’d normally install the GUI Emacs application via
brew cask install emacs
The problem is that this installs the standard GNU Emacs leveraging some basic Cocoa GUI, which results in the Magnet application not being able to snap Emacs to any of its grids. If you look at the Magnet dropdown with Emacs focused, all of the options will be greyed out.
Installing this version can be done via
brew cask install railwaycat/emacsmacport/emacs-mac
As of writing this note, Emacs 26.3 is installed and works well with Magnet 2.4.5 on Mac OS X 10.15.3.