Ignore non-opaque WebP background when saving as GIF

Tests/test_file_gif.py

@@ -859,14 +859,23 @@ def test_background(tmp_path):
     im.info["background"] = 1
     im.save(out)
     with Image.open(out) as reread:
-
         assert reread.info["background"] == im.info["background"]
 
+
+def test_webp_background(tmp_path):
+    out = str(tmp_path / "temp.gif")
+
+    # Test opaque WebP background
     if features.check("webp") and features.check("webp_anim"):
         with Image.open("Tests/images/hopper.webp") as im:
-            assert isinstance(im.info["background"], tuple)
+            assert im.info["background"] == (255, 255, 255, 255)
             im.save(out)
 
+    # Test non-opaque WebP background
+    im = Image.new("L", (100, 100), "#000")
+    im.info["background"] = (0, 0, 0, 0)
+    im.save(out)
+
 
 def test_comment(tmp_path):
     with Image.open(TEST_GIF) as im:

src/PIL/GifImagePlugin.py

@@ -886,20 +886,23 @@ def _get_palette_bytes(im):
 def _get_background(im, info_background):
     background = 0
     if info_background:
-        background = info_background
-        if isinstance(background, tuple):
+        if isinstance(info_background, tuple):
             # WebPImagePlugin stores an RGBA value in info["background"]
             # So it must be converted to the same format as GifImagePlugin's
             # info["background"] - a global color table index
             try:
-                background = im.palette.getcolor(background, im)
+                background = im.palette.getcolor(info_background, im)
             except ValueError as e:
-                if str(e) == "cannot allocate more than 256 colors":
+                if str(e) not in (
                     # If all 256 colors are in use,
                     # then there is no need for the background color
-                    return 0
-                else:
+                    "cannot allocate more than 256 colors",
+                    # Ignore non-opaque WebP background
+                    "cannot add non-opaque RGBA color to RGB palette",
+                ):
                     raise
+        else:
+            background = info_background
     return background