Setting default gVim font via `vimrc`, conditional on the operating system

I can place the following code into my _vimrc file on Windows 7/10 and the font will default to 'Consolas - size 11' when I launch gVim.

if has("gui_running")
  if has("gui_gtk2")
    set guifont=Ubuntu Mono\ 15
  elseif has("gui_macvim")
    set guifont=Menlo\ Regular:h14
  elseif has("gui_win32")
    set guifont=Consolas:h11:cANSI
  endif
endif

If I place this same code into my Ubuntu 18.04 vimrc file I don't get the expected 'Ubuntu Mono - size 15'. I just get the default font and size, as if my new vimrc code did nothing.

What do I need to change in my code?