To enlarge to square all images:

1. Copy this GIMP python-fu script:

#!/usr/bin/env python

from gimpfu import *

def square_all_images(dummy_image, dummy_drawable):
    pdb.gimp_context_push()
    # Get the list of all opened images
    images = gimp.image_list()
    image_number = len(images)
    
    # Loop through each image
    for i in range(image_number):
        image = images[i]
        drawable = image.active_layer
        pdb.gimp_progress_update((i * 1.0) / image_number)
        pdb.gimp_progress_set_text("Processing image #" + str(i + 1) + " over " + str(image_number) + "...")

        # Get image size
        width = image.width
        height = image.height

        # Calculate the maximum dimension
        max_dim = max(width, height)

        # Calculate the new dimensions
        new_width = max_dim
        new_height = max_dim

        # Calculate the offset
        offset_x = (max_dim - width) // 2
        offset_y = (max_dim - height) // 2

        # Enlarge the canvas
        image.resize(new_width, new_height, offset_x, offset_y)
        drawable.resize(new_width, new_height, offset_x, offset_y)

        # Save the image file
        filename = pdb.gimp_image_get_filename(image)
        pdb.gimp_file_save(image, drawable, filename, '?')
    
    # Refresh the image to see the changes
    pdb.gimp_context_pop()
    pdb.gimp_displays_flush()

# Define the parameters for the script
register(
    "python_fu_square_all_images",
    "Curves on all images",
    "Curves on all images and save; identical values will be ignored",
    "Fabrice TIERCELIN",
    "Fabrice TIERCELIN",
    "2024",
    "<Image>/Filters/Enlarge to square all images",
    "*",
    [],
    [],
    square_all_images
)

# Main function
main()

2. Paste it into a text file
3. Save it with a .py extension
4. Put this file on your C:\Users\YourName\AppData\Roaming\GIMP\2.10\plug-ins folder (more information below if it doesn't work)
5. Start GIMP
6. Open all your image files by drag and drop
7. Go on Filter → Enlarge to square all images
8. Launch the script → all the files are already saved as square
9. Quit GIMP ignoring the prompts

For systems other than Windows, look at the settings in GIMP:

1. Click on Edit
2. Click on Preferences
3. Go on left pane; at bottom
4. Click on Folders
5. Click on Plug-ins
6. Click on Add a new folder

Successfully tested on GIMP 2.10.36 on Windows 10.