Convert subsequent frames of L mode GIF to LA if transparency is present

Tests/test_file_gif.py

@@ -59,6 +59,17 @@ def test_invalid_file():
         GifImagePlugin.GifImageFile(invalid_file)
 
 
+def test_l_mode_transparency():
+    with Image.open("Tests/images/no_palette_with_transparency.gif") as im:
+        assert im.mode == "L"
+        assert im.load()[0, 0] == 0
+        assert im.info["transparency"] == 255
+
+        im.seek(1)
+        assert im.mode == "LA"
+        assert im.load()[0, 0] == (0, 255)
+
+
 def test_optimize():
     def test_grayscale(optimize):
         im = Image.new("L", (1, 1), 0)

src/PIL/GifImagePlugin.py

@@ -167,9 +167,15 @@ class GifImageFile(ImageFile.ImageFile):
         if self.__frame == 1:
             self.pyaccess = None
             if "transparency" in self.info:
-                self.mode = "RGBA"
+                if self.mode == "P":
                     self.im.putpalettealpha(self.info["transparency"], 0)
                     self.im = self.im.convert("RGBA", Image.Dither.FLOYDSTEINBERG)
+                    self.mode = "RGBA"
+                else:
+                    self.im = self.im.convert_transparent(
+                        "LA", self.info["transparency"]
+                    )
+                    self.mode = "LA"
 
                 del self.info["transparency"]
             else:
@@ -368,8 +374,11 @@ class GifImageFile(ImageFile.ImageFile):
         if self.__frame == 0:
             return
         if self._frame_transparency is not None:
+            if self.mode == "P":
                 self.im.putpalettealpha(self._frame_transparency, 0)
                 frame_im = self.im.convert("RGBA")
+            else:
+                frame_im = self.im.convert_transparent("LA", self._frame_transparency)
         else:
             frame_im = self.im.convert("RGB")
         frame_im = self._crop(frame_im, self.dispose_extent)