Use .xy instead of vec2() for extracting the vec2 from the vec4

frag.frag

@@ -3,5 +3,5 @@
 layout(location = 0) out vec4 outColor;
 
 void main() {
-  outColor = vec4(vec2(gl_FragCoord)/vec2(1920., 1080.), 1.0, 1.0);
+  outColor = vec4(gl_FragCoord.xy/vec2(1920., 1080.), 1.0, 1.0);
 }