Hmm, are you using Tiled 0.18.0 or 0.18.1? I have added support for exporting horizontally and vertically flipped tiles to GMS rooms in 0.18.1 (based on the
scaleY attributes). Support for exporting stretched tiles (possible in Tiled only on Object layers) was also added in Tiled 0.18.1.
I do not know of any tile rotation support in GMS. As far as I could see, rotation is only possible for object instances. To use those, you need to set the "Type" property of the object in Tiled and have an object with exactly that name in your GMS project.
I'm glad to hear that! Indeed in the limited time I've spent with GMS 1.4, I've been very surprised at the incredibly bad room editor. No wonder there has been interest in using Tiled maps in GM since years (and I wish I had looked into this sooner).
In GMS 2 the room editor has fortunately been vastly improved, but I plan to look into exporting to it eventually. Unfortunately, the file format has changed entirely (though this is also a good thing, since the new format is way more efficient).