diff -r 9e59cb403efa Lib/gzip.py
--- a/Lib/gzip.py	Tue Sep 27 11:37:10 2016 +0300
+++ b/Lib/gzip.py	Tue Sep 27 17:00:17 2016 +0300
@@ -167,6 +167,10 @@ class GzipFile(_compression.BaseStream):
                 filename = ''
         if mode is None:
             mode = getattr(fileobj, 'mode', 'rb')
+            if not mode.startswith('r'):
+                import warnings
+                warnings.warn("specify explicit mode for writing",
+                              DeprecationWarning, 2)
 
         if mode.startswith('r'):
             self.mode = READ
diff -r 9e59cb403efa Lib/test/test_gzip.py
--- a/Lib/test/test_gzip.py	Tue Sep 27 11:37:10 2016 +0300
+++ b/Lib/test/test_gzip.py	Tue Sep 27 17:00:17 2016 +0300
@@ -418,6 +418,30 @@ class TestGzip(BaseTest):
             with gzip.GzipFile(fileobj=f, mode="w") as g:
                 pass
 
+    def test_fileobj_mode(self):
+        gzip.GzipFile(self.filename, "wb").close()
+        with open(self.filename, "r+b") as f:
+            with gzip.GzipFile(fileobj=f, mode='r') as g:
+                self.assertEqual(g.mode, gzip.READ)
+            with gzip.GzipFile(fileobj=f, mode='w') as g:
+                self.assertEqual(g.mode, gzip.WRITE)
+            with gzip.GzipFile(fileobj=f, mode='a') as g:
+                self.assertEqual(g.mode, gzip.WRITE)
+            with gzip.GzipFile(fileobj=f, mode='x') as g:
+                self.assertEqual(g.mode, gzip.WRITE)
+        for mode in "rb", "r+b":
+            with open(self.filename, mode) as f:
+                with gzip.GzipFile(fileobj=f) as g:
+                    self.assertEqual(g.mode, gzip.READ)
+        for mode in "wb", "ab", "xb":
+            if "x" in mode:
+                support.unlink(self.filename)
+            with open(self.filename, mode) as f:
+                with self.assertWarns(DeprecationWarning):
+                    g = gzip.GzipFile(fileobj=f)
+                with g:
+                    self.assertEqual(g.mode, gzip.WRITE)
+
     def test_bytes_filename(self):
         str_filename = self.filename
         try: