Solution
= Solution
The <determinant> is continuous and $GL(n)=\det^{-1}(\mathbb R\setminus\{0\})$ is open in $M(n)\simeq\mathbb R^{n^2}$. Every open subset inherits the ambient smooth charts, so it is a <submanifold> and
$$
\boxed{\dim GL(n)=n^2.}
$$