The adjective 'gentle' can form its comparative and superlative degrees in two ways: by adding '-er' and '-est' (gentler, gentlest) or by using 'more' and 'most' (more gentle, most gentle). Both forms are grammatically correct, although 'gentler' and 'gentlest' are generally more common in informal speech and writing. The choice between the two often comes down to personal preference or the rhythm of the sentence.