Actually since Tiled 0.18 it’s really almost that easy, since it natively supports exporting maps to .room.gmx files. A few fixes and improvements to this were also made in Tiled 0.18.1 (and for the upcoming Tiled 0.18.2, support for image collection tilesets is pending).
The part that Tiled doesn’t do currently, is to export anything else but the map. So you need to set up your tilesets and objects in GMS as well, with the same names as used in Tiled (for objects, the “Type” property refers to the object name, whereas “Name” becomes the name of the instance). The exported .room.gmx file should then work fine.