Unfortunately I'm not a real programmer so there's no way I could write a plugin for Tiled, but I made a little application that takes the csv from the .tmx and converts it into the GMS2 one. It's very VERY basic (seriously I have no idea what I'm doing) and it's made in Construct 2 (the only tool I know how to use properly) but it does the job as long as you only have regular/rotated/mirrored tiles.
Here's the link:
(Downloads are in the release folder)
This app takes the csv tile by tile, converts it to hex and makes the alterations, then back to decimal values. I've also included the .capx and anyone with a free C2 license can take a look but like I said, it's very hacky.
Oh and the main reason I want to use Tiled is so I can work with other people who don't own a GMS2 license. This way they can use Tiled to work on the levels while I focus on the programming, and when they're done I can just import their work onto the game myself.
I do like the tilemap editor in GMS2 but it's also very handy to be able to edit it with Tiled without having to download GMS2 and opening the whole project, too.
By the way, thanks for this amazing tool! Hope someone makes on a proper GMS2 export.