You can use any vector software to lay out blocks representing tiles, doors and other in-game objects.
But then again you could do exactly the same thing on the GM room editor, and also immediately test out the level.
For coming up with ideas for level layouts, nothing beats the old fashioned pen and paper method.
You don't even need to be an artist.
That way, I can take a break from looking at Gamemaker, play some music and focus purely on layout.