Hey Armin ! I figured it out in the end :
First I think there was an issue with the fact that I was using a space in the name of the font. Then I only used .woff2 as mentioned here. Here's the code input now for people that need it. This is both in the custom CSS and the file.css supplied to the Add by "<link>" tag and CSS webfonts option of LayTheme.
@font-face {
font-family: 'GTAmerica';
src: url('/wp-content/themes/fonts/gtamerica/GT-America-Standard-Ultra-Light.woff2') format('woff2');
font-weight: 100;
font-style: normal;
font-display: swap;
}
@font-face {
font-family: 'GTAmerica';
src: url('/wp-content/themes/fonts/gtamerica/GT-America-Standard-Thin.woff2') format('woff2');
font-weight: 200;
font-style: normal;
font-display: swap;
}
etc. for the rest of the family fonts.
Then in the import options :
Font name : GTAmerica
Link tag : <link href="https://triplesine.com/wp-content/fonts/gtamerica/gtamerica.css" rel="stylesheet" type="text/css">
"font-family" CSS: font-family:'GTAmerica';
Maybe this can be added as a mini tutorial on the Lay website ?
Thanks to everyone at Lay Theme for this great theme : )