They are very similar, but they are distinct cultivars. 'Williams' is a shorter tall Cavendish variety, and 'Gran Nain' is taller dwarf Cavendish. The fruit size and bunch size are very similar, and the fruit pretty much indistinguishable.
When they fruit, you can tell because 'Gran Nain' has persistent or semi-persistent male flowers and bracts, and 'Williams' has non-persistent male flowers and bracts.
'Gran Nain'
'Williams'
