https://codereview.chromium.org/1830973002 fixed this bug. Re-enable the

dm/DM.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "CrashHandler.h"
 #include "DMJsonWriter.h"
 #include "DMSrcSink.h"
@@ skipping 571 matching lines @@
 
     // There is not currently a reason to test RAW images on image generator.
     // If we want to enable these tests, we will need to fix skbug.com/5079.
     for (const char* ext : rawExts) {
         if (path.endsWith(ext)) {
             return;
         }
     }
 
     // Push image generator GPU test.
-    // FIXME: The gpu backend does not draw kGray sources correctly. (skbug.com/4822)
-    if (kGray_8_SkColorType != codec->getInfo().colorType()) {
-        push_image_gen_src(path, ImageGenSrc::kCodec_Mode, codec->getInfo().alphaType(), true);
-    }
+    push_image_gen_src(path, ImageGenSrc::kCodec_Mode, codec->getInfo().alphaType(), true);
 
     // Push image generator CPU tests.
     for (SkAlphaType alphaType : alphaModes) {
         push_image_gen_src(path, ImageGenSrc::kCodec_Mode, alphaType, false);
 
 #if defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_IOS)
         if (kWEBP_SkEncodedFormat != codec->getEncodedFormat() &&
             kWBMP_SkEncodedFormat != codec->getEncodedFormat() &&
             kUnpremul_SkAlphaType != alphaType)
         {
@@ skipping 890 matching lines @@
                                                            Reporter* reporter,
                                                            GrContextFactory* factory);
 } // namespace skiatest
 
 #if !defined(SK_BUILD_FOR_IOS)
 int main(int argc, char** argv) {
     SkCommandLineFlags::Parse(argc, argv);
     return dm_main();
 }
 #endif