$\gamma$ is the glide angle, not the aircraft pitch angle. Other than that, the formula is correct.
For the purposes of determining the gliding capabilities of an aircraft, it is assumed that there is no vertical motion in the freestream air (which would otherwise alter the glide angle). Under such conditions, $\gamma$ will always be a positive angle - it will never be zero or negative.
In practice, there will always be some vertical motion in the freestream air, which will improve or worsen the glide angle. Specifically with strong up-currents, it is possible for the glide angle to be zero (level flight) or even negative (climb). In such cases, the aircraft would be able to stay airborne indefinitely, so using the term 'glide ratio' would be meaningless.