diff --git a/gclient_utils.py b/gclient_utils.py
index 1777cfd73..655e5aa1b 100644
--- a/gclient_utils.py
+++ b/gclient_utils.py
@@ -650,6 +650,37 @@ def GetMacWinOrLinux():
   raise Error('Unknown platform: ' + sys.platform)
 
 
+def GetBuildtoolsPath():
+  """Returns the full path to the buildtools directory.
+  This is based on the root of the checkout containing the current directory."""
+  gclient_root = FindGclientRoot(os.getcwd())
+  if not gclient_root:
+    return None
+  return os.path.join(gclient_root, 'src', 'buildtools')
+
+
+def GetBuildtoolsPlatformBinaryPath():
+  """Returns the full path to the binary directory for the current platform."""
+  # Mac and Windows just have one directory, Linux has two according to whether
+  # it's 32 or 64 bits.
+  buildtools_path = GetBuildtoolsPath()
+  if not buildtools_path:
+    return None
+
+  if sys.platform.startswith(('cygwin', 'win')):
+    subdir = 'win'
+  elif sys.platform == 'darwin':
+    subdir = 'mac'
+  elif sys.platform.startswith('linux'):
+    if sys.maxsize > 2**32:
+      subdir = 'linux64'
+    else:
+      subdir = 'linux32'
+  else:
+    raise Error('Unknown platform: ' + sys.platform)
+  return os.path.join(buildtools_path, subdir)
+
+
 def GetExeSuffix():
   """Returns '' or '.exe' depending on how executables work on this platform."""
   if sys.platform.startswith(('cygwin', 'win')):
diff --git a/gn.py b/gn.py
index ab08b5200..e3bcc9783 100755
--- a/gn.py
+++ b/gn.py
@@ -18,32 +18,15 @@ import subprocess
 import sys
 
 
-def RunGN(sourceroot):
-  # The binaries in platform-specific subdirectories in src/tools/gn/bin.
-  gnpath = os.path.join(sourceroot,
-                        'tools', 'gn', 'bin', gclient_utils.GetMacWinOrLinux(),
-                        'gn' + gclient_utils.GetExeSuffix())
-  return subprocess.call([gnpath] + sys.argv[1:])
-
-
 def main(args):
-  for arg in sys.argv:
-    if arg.startswith('--root='):
-      sourceroot = arg.replace('--root=', '')
-      dotfile_path = os.path.join(sourceroot, '.gn')
-      if not os.path.exists(dotfile_path):
-        print >> sys.stderr, 'gn.py: "%s" not found, exiting.' % dotfile_path
-        sys.exit(1)
-      return RunGN(sourceroot)
-
-  sourceroot = gclient_utils.FindFileUpwards('.gn')
-  if not sourceroot:
-    print >> sys.stderr, ('gn.py: No .gn file found in any parent of '
-                          'the current path.')
-    print >> sys.stderr, ('\nYou need to either be inside a checkout, '
-                          'or use --root to specify the checkout root.')
+  bin_path = gclient_utils.GetBuildtoolsPlatformBinaryPath()
+  if not bin_path:
+    print >> sys.stderr, ('gn.py: Could not find checkout in any parent of '
+                          'the current path.\nThis must be run inside a '
+                          'checkout.')
     sys.exit(1)
-  return RunGN(sourceroot)
+  gn_path = os.path.join(bin_path, 'gn' + gclient_utils.GetExeSuffix())
+  return subprocess.call([gn_path] + sys.argv[1:])
 
 
 if __name__ == '__main__':