diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index ded3197..6fc0dcc 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -55,6 +55,10 @@
 
 " In the GUI we can always change the screen size.
 if has('gui_running')
+  if has('gui_gtk')
+    " to keep screendump size unchanged
+    set guifont=Monospace\ 10
+  endif
   set columns=80 lines=25
 endif