I think you try to consider too many constraints at the same time.
The montage filter in G’MIC is not able to target a desired aspect ratio. It actually doesn’t modify the aspect ratio of the input images but resize them according to the specified montage scheme.
For instance, if you try to create an horizontal montage from two 2:3 images, of size 2000x3000 (portrait) and 3000x2000 (landscape), with layout “H(0,1)”, you’ll get a result of size 4333x2000, which is absolutely not 2:3.
Same if you append two horizontal montages like this, but vertically. You’ll get a 4333x4000 image, still not 2:3.
I guess that you’ll probably have to crop your images in a way or another to ensure you get the desired aspect ratio. And honestly, knowing exactly what crop coordinates to use is probably not easy to guess 