Since there is no solution for non-unix (for now), remove the condition

src/Composer/Downloader/GzipDownloader.php

@@ -36,10 +36,6 @@ class GzipDownloader extends ArchiveDownloader
 
     protected function extract($file, $path)
     {
-        $processError = null;
-
-        // Try to use gunzip on *nix
-        if (!defined('PHP_WINDOWS_VERSION_BUILD')) {
         $targetFile = $path . '/' . basename(substr($file, 0, -3));
         $command = 'gzip -cd ' . escapeshellarg($file) . ' > ' . escapeshellarg($targetFile);
 
@@ -50,7 +46,6 @@ class GzipDownloader extends ArchiveDownloader
         $processError = 'Failed to execute ' . $command . "\n\n" . $this->process->getErrorOutput();
         throw new \RuntimeException($processError);
     }
-    }
 
     /**
      * {@inheritdoc}