Gmsh Icon
This is minor bug. Gmsh icon is stored in: "/usr/share/pixmaps/gmsh_32x32.xpm"
The standard paths to icons in UNIX-like systems are: "/usr/share/icons/hicolor/" or "~/local/share/icons/hicolor/".
Because of this custom themes cannot change this icon. For example, Numix Icon Theme got a custom icon for Gmsh and cannot set it by default. There is an issue created here if you want to check it (https://github.com/numixproject/numix-icon-theme-circle/issues/1231).