sil-gentium-basic-fonts/59-sil-gentium-basic-book.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
<!-- SPDX-License-Identifier: MIT -->
<fontconfig>
  <!-- Generic name assignment -->
  <alias>
    <family>Gentium Book Basic</family>
    <default>
      <family>serif</family>
    </default>
  </alias>
  <!-- Generic name aliasing -->
  <alias>
    <family>serif</family>
    <prefer>
      <family>Gentium Book Basic</family>
    </prefer>
  </alias>
</fontconfig>