documentclass{article}
usepackage{amsmath}
begin{document}

$G$, $P$, $Z$, $R$, $T$ are all some variables.

Default:
begin{equation*}
    a = frac{8GP}{ZRT}
end{equation*}

Small space (textbackslash!):
begin{equation*}
    a = frac{8G!P}{Z!RT}
end{equation*}

Large space (textbackslash,):
begin{equation*}
    a = frac{8,GP}{ZR,T}
end{equation*}
end{document}