defining texture gather to error on unsuported hardware.

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/GLCore.hlsl

 #define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                textureName.Gather(samplerName, coord3)
 #define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   textureName.Gather(samplerName, float4(coord3, index))
 #else
-#define GATHER_TEXTURE2D(textureName, samplerName, coord2)                  SAMPLE_TEXTURE2D(textureName, samplerName, coord2).rrrr
-#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)     SAMPLE_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index).rrrr
-#define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                SAMPLE_TEXTURECUBE(textureName, samplerName, coord3).rrrr
-#define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   SAMPLE_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index).rrrr
+#define GATHER_TEXTURE2D(textureName, samplerName, coord2)                  ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURE2D)
+#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)     ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURE2D_ARRAY)
+#define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURECUBE)
+#define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURECUBE_ARRAY)
 #endif
 

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/GLES2.hlsl

 #define LOAD_TEXTURE2D_ARRAY_LOD(textureName, unCoord2, index, lod) half4(0, 0, 0, 0) // Not supported
 
 // Gather not supported. Fallback to regular texture sampling.
-#define GATHER_TEXTURE2D(textureName, samplerName, coord2)                  SAMPLE_TEXTURE2D(textureName, samplerName, coord2).rrrr
-#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)     SAMPLE_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index).rrrr
-#define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                SAMPLE_TEXTURECUBE(textureName, samplerName, coord3).rrrr
-#define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   SAMPLE_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index).rrrr
+#define GATHER_TEXTURE2D(textureName, samplerName, coord2)                  ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURE2D)
+#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)     ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURE2D_ARRAY)
+#define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURECUBE)
+#define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURECUBE_ARRAY)

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/GLES3.hlsl

 #define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                textureName.Gather(samplerName, coord3)
 #define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   textureName.Gather(samplerName, float4(coord3, index))
 #else
-#define GATHER_TEXTURE2D(textureName, samplerName, coord2)                  SAMPLE_TEXTURE2D(textureName, samplerName, coord2).rrrr
-#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)     SAMPLE_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index).rrrr
-#define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                SAMPLE_TEXTURECUBE(textureName, samplerName, coord3).rrrr
-#define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   SAMPLE_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index).rrrr
+#define GATHER_TEXTURE2D(textureName, samplerName, coord2)                  ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURE2D)
+#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)     ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURE2D_ARRAY)
+#define GATHER_TEXTURECUBE(textureName, samplerName, coord3)                ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURECUBE)
+#define GATHER_TEXTURECUBE_ARRAY(textureName, samplerName, coord3, index)   ERROR_ON_UNSUPPORTED_FUNCTION(GATHER_TEXTURECUBE_ARRAY)
 #endif