Importing into Unity is a joy. When I say there's nothing to it, that's quite literally true. Unity automatically imports anything you save or move into its asset directory. So it’s just a matter of finding where that is, and then always saving your exported files there.

The good thing about Unity is that it automatically checks for changes and updates itself with new versions of your asset files. Say you don't like the grass texture you used after all? Just change it in GIMP and export a copy of the image directly into the texture folder in Unity. Presto, the grass on your terrain changes too!