If I don’t use Mono or Gnome, what else from him is part of my stack today?
Unity engine , the most popular game engine based on mono. Also gnome software like glib , cairo , harfbuzz prolly used as foundation by all OSes, gui toolkit, programming language or browser on the market.
It's likely you do use Mono or a successor. The modern dotnet framework is a descendent of Mono and is used in a variety of websites, games, and other applications.