Create a collage/montage using G'MIC under GIMP

Okay, sorry, I was unclear:

The most important question is:

I have a target format - I want to have a horizontal graphic in 2:3 aspect ratio that I want to print as a photo print. So the result should be a picture with e.g. the size 2000x3000 pixels, or 4000x6000 pixels.

I have 4 photos (in high resolution) that I want to place on it - two are upright and two are landscape. (And /somewhere/ I want to have space to write a text - this could be on another small image layer I place where there is space).

The arrangement is ideally like this:

landscape|upright
upright|landscape

And possibly to the right (or left) of it a layer from top to bottom for the text. If that works. If not, then I leave the text out of this and put it into the final image.

At first I only experimented with the 4 photos. I managed to arrange the photos in the desired order horizontally and vertically. But what is completely unclear is how I can influence the result in terms of the size of the pictures and the size of the final result.

When I place the 4 images, the result is almost square. That would be OK in principle, because I could then place the text layer on the side. But at the moment this is all completely random to me.

BR,
Eriol