This has anime-related meme templates, so it counts!

I haven’t done this in a while so things might have changed, but if you’re working in Impress with one segment per slide and you want the whole slide, you can export your slides as HTML and it’ll create a folder with an HTML file and some images. You can discard the HTML and keep the images, which you can then open as layers and assemble in GIMP (or use ImageMagick’s montage to do the same from the command line). Alternatively, perhaps even easier, you can export the presentation as a PDF and import the PDF as layers into GIMP (make sure to set the size that you want), which you can then arrange as before.

For arranging layers in GIMP more easily, you can try to find a sprite sheet plugin.

Though reading your post again, it looks like you probably don’t want the whole slide. But with memes of this format, it’s probably easiest to do it entirely in Impress and then assemble afterwards. You can set the page size to be any arbitrary page size. LibreOffice Draw is probably the more suitable program compared to Impress, but it doesn’t matter too much.

Doing everything entirely in GIMP is also an option. Image > Fit Canvas to Layers is your friend.

As for my workflow, I create the template in GIMP and then add text on top with Aegisub. I find it much easier to manipulate text there rather than in GIMP.