These are two useful LaTeX macros for creating equality-at and, more generally, relation-at signs. These macros depend on the mathtools package. As with all other macros, you should add them to your preamble in order to use them.
The general macro is relat. It takes two arguments: the relation and an expression where the relation takes place (the “at”). The equality-at macro, eqat, is a specific case of relat. I’ve created it because it is commonly used and only requires passing the “at” argument.
Here is the LaTeX code for the macros.
newcommand{relat}[2]{mathrel{bigg|_{mathrlap{#2}}mathrlap{#1}hphantom{{}_{#2}}}}
newcommand{eqat}[1]{mathrel{bigg|_{mathrlap{#1}}mathrlap{=}hphantom{{}_{#1}}}}
This is a basic example of how to use the macros. It should also give you an idea of how they look.
documentclass{article}
pagestyle{empty}
usepackage{mathtools}
newcommand{relat}[2]{mathrel{bigg|_{mathrlap{#2}}mathrlap{#1}hphantom{{}_{#2}}}}
newcommand{eqat}[1]{mathrel{bigg|_{mathrlap{#1}}mathrlap{=}hphantom{{}_{#1}}}}
begin{document}
[
Delta=left[begin{array}{cc} P & Q\ f' & g'end{array}right]
eqat{Gamma}
left[begin{array}{cc} -2t & 4t\ 1 & -2end{array}right]
]
[
cos x relat{leq}{x=frac{pi}{3}} frac{sqrt{2}}{2}
]
end{document}
And the output for the example is
