Adding custom fonts in mosaico
What worked for me with google-fonts is the following:

After the  tag add the include path generated from google-fonts, in this example Modak and Open Sans.

  @import url('https://fonts.googleapis.com/css?family=Modak|Open+Sans');


Then in the @supports block, in the face part add the fonts separated by '|', to be included in the blocks:

  face { label: Font; widget: select; options: Arial|Impact|'Open Sans', sans-serif|Times New Roman|'Modak'}


Edit: local fonts can be included by putting them on accessible on your server, and then import them in the template like this: 

@font-face {
  font-family: 'your_font_name';
  src: url(https://example.com/files/fonts/your_font/your_font_file.ttf) format('ttf'), 
    url(https://example.com/files/fonts/your_font/your_font_file.woff2) format('woff2');
        }


Then add those to the dropdown menu in the @supports block same as above.

That way your fonts can be loaded from your server.