diff --git a/masque/file/gdsii.py b/masque/file/gdsii.py
index cd2c6c5..4dce32c 100644
--- a/masque/file/gdsii.py
+++ b/masque/file/gdsii.py
@@ -144,7 +144,7 @@ def writefile(
     with tmpfile(path) as base_stream:
         streams: tuple[Any, ...] = (base_stream,)
         if path.suffix == '.gz':
-            stream = cast(IO[bytes], gzip.GzipFile(filename='', mtime=0, fileobj=base_stream, mode='wb'))
+            stream = cast(IO[bytes], gzip.GzipFile(filename='', mtime=0, fileobj=base_stream, mode='wb', compresslevel=6))
             streams = (stream,) + streams
         else:
             stream = base_stream