Since 2014, MacTeX don't provide a systemwide installation of the TeX Gyre fonts. See this archived page: New features in MacTeX 2014
In previous years, MacTeX contained optional packages to make the Latin-Modern and TeX-Gyre fonts in TeX available to other Mac OS X programs. These are no longer provided because only a few users installed them, and because a better procedure is to create symbolic links in /Library/Fonts to desired opentype TeX fonts in TeX Live.
What you can do is copying TeX Gyre fonts from
/usr/local/texlive/2018/texmf-dist/fonts/opentype/public/tex-gyre (here is the path for MacTeX 2018, change the year if necessary) to
_yourName_ is the name of your account on your macOS session. By default, this
Library folder is hidden (for security reason, because of some inexperienced macOS users.) But this install the fonts only for your macOS session. If you have several accounts on your Mac, you can also install the fonts in
/Library/Fonts on the root of your system.
Note that if you would access via XeLaTeX to any of the truetype or opentype fonts provided by MacTeX, not only the TeX-Gyre fonts, you must copying it either in
Opentype fonts are located here (for 2018):
Truetype fonts are located here (for 2018):