Ubuntu – Which software is used in Ubuntu for supporting Gill sans font Just like MS Office in Windows

Font names in Ubuntu are managed by library named fontconfig. fontconfig has a notion of aliases; four of those aliases are sans, sans-serif, serif, and monospace. To see what actual fonts are pointed to by those aliases use the command fc-match:

$ fc-match sans-serif
DejaVuSans.ttf: "DejaVu Sans" "Book"
$ fc-match sans
DejaVuSans.ttf: "DejaVu Sans" "Book"
$ fc-match serif
DejaVuSerif.ttf: "DejaVu Serif" "Book"
$ fc-match monospace
DejaVuSansMono.ttf: "DejaVu Sans Mono" "Book"

To modify the meaning of the standard aliases sans, sans-serif, serif and monospace you must create or edit a per-user configuration file, ~/.config/fontconfig/fonts.conf (or ~/.fonts.conf, depending on fontconfig version and system configuration). (You can of course edit the system-wide configuration file, but that would be rude.) For example,

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
  <alias>
    <family>serif</family>
    <prefer><family>Liberation Serif</family></prefer>
  </alias>
  <alias>
    <family>sans-serif</family>
    <prefer><family>Liberation Sans</family></prefer>
  </alias>
  <alias>
    <family>sans</family>
    <prefer><family>Liberation Sans</family></prefer>
  </alias>
  <alias>
    <family>monospace</family>
    <prefer><family>Liberation Mono</family></prefer>
  </alias>
</fontconfig>

See a worked out example at How to Set Default Fonts on Linux on Season of Code.