lightbox on gallery enlarges the cropped image, not the original format

Do you have a link where you can see this? Another plugin or your theme may also be interfering here.

I tested this with several themes, with plugins disabled. For example with Twenty Twenty Three. Here is an expample page:
https://abraxis-kassel.de/demo-wordpress/galerie-test/
Below the gallery a few images are shown as individual images for demonstration purposes to show their original image ratio.

Can see the problem and can also reproduce it in my case. However, I don’t see any way of influencing the behaviour. I would recommend that you report this to the Gutenberg team so that they can evaluate it: https://github.com/WordPress/gutenberg/issues

Add this CSS for quick fix:

.lightbox-image-container {
    width: 100vw !important;
}
.lightbox-image-container figure img {
    width: auto !important;
    height: auto !important;
    object-fit: contain !important;
    max-height: 100% !important;
}

Thanks threadi for checking and pointing out and Ashutosh Sharma for the quick fix. Works flawlessly!