G-Buffers don't exist...

Hi, I'm trying to make a shader that adds my custom sunlight effect, so in an image effect, I need to acces the normal g-buffer (_CameraGBufferTexture2), but it doesn't exist (Unity never sets the uniform anywhere). I know this because when I try to sample it, it gives me a grey color, and when I sample a sampler that obviously wouldn't exist, it also gives me a grey color. Here's my shader code:

 Shader "Hidden/Grayscale Effect" {
 Properties {
 }
 SubShader {
     Pass {
         ZTest Always Cull Off ZWrite Off
 CGPROGRAM
 #pragma vertex vert_img
 #pragma fragment frag
 #include "UnityCG.cginc"
 uniform sampler2D _CameraGBufferTexture2;
 float4 frag (v2f_img i) : SV_Target
 {
     half4 normal=tex2D(_CameraGBufferTexture2, i.uv);
     return float4(normal.x,normal.y,normal.z,1.0);
 }
 ENDCG
     }
 }
 Fallback off
 }

I'm working off the greyscale image effect from the standard assets, so do I need to set anything else outside the shader? I'm using unity 5.4b20. Thank you :)