Fix lost transparency when converting to GIF format

tests/Feature/Gd/ConvertPngGif.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Feature\Gd;
+
+use Intervention\Image\ImageManager;
+use Intervention\Image\Tests\GdTestCase;
+
+class ConvertPngGif extends GdTestCase
+{
+    public function testConversionKeepsTransparency(): void
+    {
+        $converted = ImageManager::gd()
+            ->read(
+                $this->readTestImage('circle.png')->toGif()
+            );
+
+        $this->assertTransparency($converted->pickColor(0, 0));
+        $this->assertColor(4, 2, 4, 255, $converted->pickColor(25, 25), 4);
+    }
+}