Use macro for depth down sample and support when gather red is not available

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/API/D3D11.hlsl

 #define LOAD_TEXTURE3D(textureName, unCoord3)                       textureName.Load(int4(unCoord3, 0))
 #define LOAD_TEXTURE3D_LOD(textureName, unCoord3, lod)              textureName.Load(int4(unCoord3, lod))
 
-#define GATHER_TEXTURE2D(textureName, samplerName, coord2)                textureName.Gather(samplerName, coord2)
-#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)   textureName.Gather(samplerName, float3(coord2, index))
+#define PLATFORM_SUPPORT_GATHER
+#define GATHER_TEXTURE2D(textureName, samplerName, coord2, offset)        textureName.Gather(samplerName, coord2, offset)
+#define GATHER_TEXTURE2D_ARRAY(textureName, samplerName, coord2, index)   textureName.Gather(samplerName, float3(coord2, index), offset)
-#define GATHER_RED_TEXTURE2D(textureName, samplerName, coord2)              textureName.GatherRed(samplerName, coord2)
-#define GATHER_GREEN_TEXTURE2D(textureName, samplerName, coord2)            textureName.GatherGreen(samplerName, coord2)
-#define GATHER_BLUE_TEXTURE2D(textureName, samplerName, coord2)             textureName.GatherBlue(samplerName, coord2)
+#define GATHER_RED_TEXTURE2D(textureName, samplerName, coord2, offset)      textureName.GatherRed(samplerName, coord2, offset)
+#define GATHER_GREEN_TEXTURE2D(textureName, samplerName, coord2, offset)    textureName.GatherGreen(samplerName, coord2, offset)
+#define GATHER_BLUE_TEXTURE2D(textureName, samplerName, coord2, offset)     textureName.GatherBlue(samplerName, coord2, offset)

ScriptableRenderPipeline/HDRenderPipeline/HDRP/RenderPipelineResources/DepthDownsample.compute

 
     // Downsample the block
     float2 offset = float2(threadUL) * 2.0f + 1.0f;
-    float4 depths = _Source.GatherRed(sampler_PointClamp, offset * _SrcSize.zw, 0.0);
+#if defined(PLATFORM_SUPPORT_GATHER)
+    float4 depths = GATHER_RED_TEXTURE2D(_Source, sampler_PointClamp, offset * _SrcSize.zw, 0.0);
+#else
+    // Downsample the block
+    float p00 = SAMPLE_TEXTURE2D_LOD(_Source, sampler_PointClamp, (offset) * _SrcSize.zw, 0.0).x;
+    float p10 = SAMPLE_TEXTURE2D_LOD(_Source, sampler_PointClamp, (offset + float2(1.0, 0.0)) * _SrcSize.zw, 0.0).x;
+    float p01 = SAMPLE_TEXTURE2D_LOD(_Source, sampler_PointClamp, (offset + float2(0.0, 1.0)) * _SrcSize.zw, 0.0).x;
+    float p11 = SAMPLE_TEXTURE2D_LOD(_Source, sampler_PointClamp, (offset + float2(1.0, 1.0)) * _SrcSize.zw, 0.0).x;
+    float4 depths = float4(p00, p10, p01, p11);
+#endif
 
     float minDepth = min(min(depths.x, depths.y), min(depths.z, depths.w));