shadertest | A Python library for unit testing GLSL functions | Unit Testing library

In the implementation of the shader, the value for sigma has to be in range [0.0, 1.0]. A radius of 30 seems to be to large. Start with u_sigma = 0.5 and u_radius = 10.

The the shader effect still looks wrong, because the computation of the alpha channel is wrong.
You have copied the shader form a WebGL. WebGL by default uses premultiplied alpha, but your desktop application does not. See Straight versus premultiplied.
Skip the division of the weighted sum of the color channels by the weighted sum of the alpha channel:

gaussCol.rgb = clamp( gaussCol.rgb / gaussCol.w, 0.0, 1.0 );

That FullscreenQuad class uses a_texCoord0 for the UVs name, but the shader from the tutorial uses a_texCoords. These names need to match.

To avoid difficult-to-debug problems like this, I would recommend not turning off ShaderProgram.pedantic while doing development builds. But do use it for releases to avoid hard crashes.

Unrelated to your issue: The tutorial's last three lines in the fragment shader put a lot of hope in non-guaranteed compile-time optimizations. GLSL already has an optimized linear interpolation function. It might also be faster to only calculate RGB, since alpha will be unused in the end. The last lines should be changed to something like this.

I don't know how to work around low percision issues except to change your math not to run into the but you can conditionally ask for high percision