Roman snail. The shell is gray to brownish-yellow in color. Young individuals have a nut-brown shell and older have a brighter grey-yellow shell. The shell changes color as the cochlea dedicate more and more calcium in the shell during its lifetime. A Roman snail can be 30 years old but usually they become around 10 years in the wild.
So this means the snail on this picture is an older one. At the moment we have a lot of Roman snails in our garden.