usepackage{graphicx}
usepackage{subcaption}
usepackage{gnuplottex}

begin{figure}[h!]
begin{subfigure} [b]{0.50textwidth}
centering
 fontsize{8}{8}selectfont
 begin{gnuplot}
  set terminal epslatex size 3,2
  set grid
  set xlabel "X"
  set ylabel "Y $_a$"
  set xrange [0:5]
  set yrange [0:25]
  plot x**2 title "x$^2$" w lp lc 8 pt 5 ps 1.1 lw 2
 end{gnuplot}
 caption{}
end{subfigure}
begin{subfigure} [b]{0.5textwidth}
centering
 fontsize{8}{8}selectfont
 begin{gnuplot}
  set terminal epslatex size 3,2
  set grid
  set xlabel "X"
  set ylabel "Y $_a$"
  set xrange [0:5]
  set yrange [0:25]
  plot x**2 title "x$^2$" w lp lc 8 pt 5 ps 1.1 lw 2
 end{gnuplot}
 caption{}
end{subfigure}
caption{blabla}
end{figure}