The image is 840 x 1427 pix. That is on the small size and not wonderful quality. Do you have a larger version ? To fit to A4 portrait @ 300 ppi the image will scale almost times four.
However, using what is there, I might use a workflow such as:
Open a new image using the standard Gimp A4 template File -> New -> Template -> A4
Add the image as a layer File -> Open as Layers
Use the Scale tool (shift-S) and the background layer as a guide to scale the layer as a 'fit' over the A4 background.
That leaves a layer larger than the canvas so a tidy up Image -> Fit Canvas to Layers then Layer -> Layer to Image Size then Image -> Zealous Crop Result should be an image with a size around 3000 x 5000 pix.
Splitting for printing:
Set Guides at 33.3 and 66.7 percent. Image -> Guides -> New Guide (by percent)
Gimp 2.10 Divide the image into three new images (for printing) Image -> Slice Using Guides .
Gimp 2.8 Use Image -> Transform -> Guillotine
Use each of those new images to print on separate A4 sheets. You will need to trim the margin's before glue-ing
All that in a 5 minute video: https://youtu.be/TMANEkM_iw4
Remember to Save your work as you go along, mishaps do occur.
The Windows Gimp print is a bit flaky, sometimes best to use some other application for printing.
Of course there are always alternatives, https://posterazor.sourceforge.io/ specifically made for such work.
best of luck.
However, using what is there, I might use a workflow such as:
Open a new image using the standard Gimp A4 template File -> New -> Template -> A4
Add the image as a layer File -> Open as Layers
Use the Scale tool (shift-S) and the background layer as a guide to scale the layer as a 'fit' over the A4 background.
That leaves a layer larger than the canvas so a tidy up Image -> Fit Canvas to Layers then Layer -> Layer to Image Size then Image -> Zealous Crop Result should be an image with a size around 3000 x 5000 pix.
Splitting for printing:
Set Guides at 33.3 and 66.7 percent. Image -> Guides -> New Guide (by percent)
Gimp 2.10 Divide the image into three new images (for printing) Image -> Slice Using Guides .
Gimp 2.8 Use Image -> Transform -> Guillotine
Use each of those new images to print on separate A4 sheets. You will need to trim the margin's before glue-ing
All that in a 5 minute video: https://youtu.be/TMANEkM_iw4
Remember to Save your work as you go along, mishaps do occur.
The Windows Gimp print is a bit flaky, sometimes best to use some other application for printing.
Of course there are always alternatives, https://posterazor.sourceforge.io/ specifically made for such work.
best of luck.