pgfplotscreateplotcyclelist{gray}{ % define a new cycle list
 {fill=black!20},
 {fill=black!40},
 {fill=black!60}
}


begin{tikzpicture}
begin{axis}[
  ymin = 0, ymax = 25,
  ybar,
  bar width=17, % added
  enlarge x limits=0.3,
  legend  style={at={(0.7 ,1)},
  anchor=north,legend  columns =-1},
  ylabel ={Gas Cost & Price in ETH},
  xlabel ={Sources Quantity},
  symbolic x coords ={7,14,21},
  xtick=data,
  nodes  near  coords ,
  nodes  near  coords  align ={vertical},
  nodes near coords style={font=tiny}, % reduce font size of nodes near coords
  cycle list name=gray, % use the new cycle list
]

addplot  coordinates  {(7 ,11) (14,15) (21,19)};
addplot  coordinates  {(7,5) (14,7) (21,9)};

 addlegendentry{Gas}
 addlegendentry{ETH}
end{axis}
end{tikzpicture}

%7:   0.11449906 ETH - 5 724 953 Gas
%14: 0.153421 ETH     - 7 671 050 Gas
%21: 0.1962195 ETH -  9 810 975 Gas