Use Atom's icon.

atom.gyp

@@ -5,6 +5,9 @@
     'app_sources': [
       'app/atom_main.cc',
     ],
+    'bundle_sources': [
+      'browser/mac/atom.icns',
+    ],
     'coffee_sources': [
       'browser/api/lib/app.coffee',
       'browser/api/lib/atom_delegate.coffee',
@@ -126,6 +129,9 @@
       'sources': [
         '<@(app_sources)',
       ],
+      'mac_bundle_resources': [
+        '<@(bundle_sources)',
+      ],
       'include_dirs': [
         '.',
       ],

browser/mac/Info.plist

@@ -6,9 +6,11 @@
 	<string>com.github.atom</string>
 	<key>CFBundleName</key>
 	<string>${PRODUCT_NAME}</string>
+  <key>CFBundleIconFile</key>
+  <string>atom.icns</string>
 	<key>NSMainNibFile</key>
 	<string>MainMenu</string>
 	<key>NSPrincipalClass</key>
-	<string>BRYApplication</string>
+	<string>AtomApplication</string>
 </dict>
 </plist>