Tangent and normal and tangent

The tangent and normal at the point $P=(at^2,2at)$ to the parabola $y^2=4ax$ meet the $x$ -axis in $T$ and $G$ respectively, then the angle at which the tangent at $P$ to the parabola is inclined to the tangent at $P$ to the circle through $P,T$ and $G$ is equal to $\text{\_\_\_\_\_\_\_\_\_\_}$ .