See Extreme Value Statistics of Eigenvalues of Gaussian Random Matrices (2008), in particular the large-$n$ result:
$$\text{Prob}(E_{\rm smallest}\geq x)\rightarrow\exp\left[-n^2\Phi\left(\frac{x+\sqrt{2n}}{\sqrt{n}}\right)\right],\;\;-\sqrt{2n}<x<0,$$ $$\Phi(z)=S(-\sqrt{2})-S(-\sqrt{2}-z),$$ $$S(z)= \frac{1}{216}\left[ 72 z^2 -2z^4 +(30 z + 2z^3) \sqrt{6 +z^2}+ 27\left( 3 + \ln 1296 - 4 \ln\left(-z + \sqrt{6 +z^2}\right)\right)\right].$$
The probability that all eigenvalues are positive follows from $x\rightarrow 0$,
$$\text{Prob}(E_{\rm smallest}\geq 0)\rightarrow3^{-n^2/4}.$$