Download All Google Fonts
Open Font Book
The next step is to open the Font Book directory on macOS (use spotlight to find this). Next, we have to reveal the location of all the fonts in macOS so that we know where it is placed. To do that right-click on any of the fonts and show in the finder. The popup menu will look like below.
Transfer Google Fonts
From the google fonts-master directory, we have to filter the fonts by extension by typing ".ttf" which is to get all files with the TrueType Font. When it's already filtered you will be looking at a font of more than 4000 files. Next, you will have to copy all of the fonts and paste them into the fonts directory. Do note that the size will be around 1GB.
What the transfer is complete do try to look for the font in your preferred text editor. Lastly, try searching for "karla" font and apply the font to the text editor. I hope the steps are straightforward enough and if there are any questions, do comment below and have fun.