Gravity waves are caused by moving objects, and the waves themselves move. So if you did see constructive interference between gravity waves, it would be temporary. Gravity waves also tend to be extremely small, small enough that it was only this year (2016) that we were finally able to detect them (at LIGO).

Here's a video of what standing waves look like in water: https://www.youtube.com/watch?v=NpEevfOU4Z8