Q:

vim powerline doesnt shows fonts in gVim

I installed powerline and downloded all the fonts.  Configured _vimrc to the font FiraMono and it worked but I can't change the font into nothing else
I installed RobotoMono in my windows But I cant set my gVim into RobotoMono font. 
When I edit _vimrc into RobotoMono then airline status bar goes weird and font become Fixedsys My _vimrc screenshot
I am using gVim on windows  searched for solution but nothing worked. 

A:

When GUI font becomes fixedsys on gvim, it means gvim cant find the font you specified (in this case, RobotoMono); so it fallbacks to its default font thats fixedsys and of course unpatched fixedsys is not contain fancy powerline symbols and as the result, airline statusbar becomes weird. 
As you noted your patched FiraMono font works, the problem seems to be with font name that you are entered in your vimrc so first from gvim menu go to: Edit > Font > show more fonts and from there select the RobotoMono font that you Installed, after selecting that font a name will appear underneath dropdown list; thats the right name you should enter in your vimrc file.  By the way i have got these patched fonts installed and the names are:

Roboto Mono for Powerline
Roboto Mono Light for Powerline
Roboto Mono Medium for Powerline
Roboto Mono Thin for Powerline

AND do not forget to scape spaces in them with \
P.S. You dont need let g:Powerline_symbols="fancy" in your vimrc.