Final product

res/shaders/gradient/frag.gls

@@ -0,0 +1,11 @@
+#version 330 core
+
+uniform vec4 input_color;
+
+in float value;
+
+out vec4 color;
+
+void main() {
+	color = input_color * vec4(value, value, value, 1);
+}

res/shaders/gradient/vert.gls

@@ -0,0 +1,13 @@
+#version 330 core
+
+uniform mat4 mvp;
+
+layout (location = 0) in vec3 vertices;
+layout (location = 1) in float in_value;
+
+out float value;
+
+void main() {
+    value = in_value;
+	gl_Position = mvp * vec4(vertices, 1);
+}