To export a file in Gravity Sketch, go to the Menu under Save you will find the following interface. Select the file format you wish to export by using the slider (OBJ, OBJ single sided, FBX or IGES). Finally, press the export button on the right and your file will be exported as the file format you selected.
To find your exported files, go to the Gravity Sketch folder on your computer and you will be able to find them inside the Exported Sketches Folder.
The following table shows the importing capabilities for the OBJ, FBX and IGES file formats in most commonly used 3D applications.