In the context of people, "kind" and "gentle" are synonyms, according to the Cambridge dictionary, so yes. However, "kind" is much more usual in this context.
Those two words are so called 'near synonyms', but there's a 'slight' difference. A kind person is someone who lets you have the last seat on the bus because you're tired, while a friendly person is someone who smiles at you when you're waiting at the bus stop, or who makes pleasant conversation with you. Being friendly is a manner, while kindness is an act.