How can I have the ground sampling distance from the images?

I'm not sure I understand "ground sampling distance" for an area, but assume you mean the pixel resolution of the image.If the images are geo-rectified, then you can just zoom in until you can see the pixels, then use a measuring tool to get the pixel width in whatever software you have. Otherwise, I think you could estimate the distance across the image and divide by the number of pixels across the image.

In (my) question and answer Georeference single drone image from EXIF data, there is a calculation for the horizontal field of view (in metres) of a UAV image with given lens characteristics, pitch, and altitude.

For pitch -90 degrees, i.e. pointing straight down, this reduces to

ground_fov_horiz = altitude * sensor_width / focal_length

If you divide this by the number of pixels in the image horizontally, you should get the GSD.

Example for a Mavic mini at altitude 100m:

ground_fov_horiz = 100m * 6.17mm / 4.36mm = 142m
GSD = 142m / 4000 pixels = 3.5 cm/pixel

Alternatively, a lot of software for UAV route planning does it for you, for instance the mapping component of dronelink.com.

You have to be a bit careful since the camera focal length is usually specified prior to any sensor lens corrections. So you may be a few percent off.