Let $M$ and $D$ denote the midpoint of $AB$ and the foot of the altitude from $C$ to $AB$, respectively, and let $r$ be the inradius of $\bigtriangleup ABC$. Since $C,G,M$ are collinear with $CM = 3GM$, the distance from $C$ to line $AB$ is $3$ times the distance from $G$ to $AB$, and the latter is $r$ since $IG \parallel AB$; hence the altitude $CD$ has length $3r$. By the double angle formula for tangent, $\frac{CD}{DB} = \tan\beta = \frac{3}{4}$, and so $DB = 4r$. Let $E$ be the point where the incircle meets $AB$; then $EB = r/\tan(\frac{\beta}{2}) = 3r$. It follows that $ED = r$, whence the incircle is tangent to the altitude $CD$. This implies that $D=A$, $ABC$ is a right triangle, and $\alpha = \frac{\pi}{2}$.