Explanations

Let’s start with a linear gradient

#define PROCESSING_COLOR_SHADER

uniform vec2 resolution;

void main( void ) {

This is our normalize position on screen (from 0 to 1 in both directions)

	vec2 position = gl_FragCoord.xy / resolution.xy;

To draw a linear horizontal gradient, we take x position on screen.

  float v = position.x;

And then make a gray scaled color from this value : same value for red, green and blue channels. We leave Alpha channel at 1 (full opacity)

	gl_FragColor = vec4(v,v,v,1.0);
}

As expected, we have a nice gradient from black (left) to white (right)

Full Code Source

#define PROCESSING_COLOR_SHADER

uniform vec2 resolution;

void main( void ) {

	vec2 position = gl_FragCoord.xy / resolution.xy;
	
  float v = position.x;

	gl_FragColor = vec4(v,v,v,1.0);
}